What is an Angiosperm?
An angiosperm is a flowering plant that produces seeds enclosed in a carpel. They are called flowering plants and are the largest as well as the most diverse group in the kingdom Plantae. These plants represent approximately 80% of all the known extant green plants. The angiosperms comprise vascular seed plants. In these plants, the ovule (egg) is fertilized and forms into a seed and the seed develops in the enclosed hollow ovary.
The ovary of angiosperms is enclosed in a flower and its flower contains the male or female reproductive organs or even both. Fruits are formed from the maturing floral organs of the angiospermous plant and fruit-bearing happens to be one of its characteristics.
Moreso, there is another large group of vascular seed plants referred to as gymnosperms. In contrast with angiosperm, their seeds do not develop enclosed within an ovary, rather it is on the surfaces of reproductive structures (cones) that their seeds are usually borne. The seeds are usually naked on the surfaces of the cones.
These flowering plants are more successful at colonizing more habitats than other groups of terrestrial plants. This is a result of the diversity of form within the angiosperms. In nonvascular plants like the bryophytes, all the cells in the plants are involved in every function needed to nourish, support, and extend the body of the plant. These cells function in the nutrition, photosynthesis, and cell division carried out by the plant. Such characteristics are different with angiosperms, as they have evolved specialized cells and tissues to carry out such functions.
Also, flowering plants further evolved specialized vascular tissues such as phloem and xylem that translocate nutrients and water to all parts of the plant body. The presence of localized regions for plant growth is also another significant evolutionary advancement that angiosperms have over the non-vascular and more primitive vascular plants.
These localized regions are called meristems and cambia. The meristem extends the length of the plant body while the cambia extend the width of the plant body. These regions, except under certain conditions, are the only areas in which mitotic cell division takes place in the body of the plant. Though cell differentiation continues to take place over the life of the plant.
Angiospermous plants have specialization evolved to enable them to adapt to the terrestrial habitat. Such specialized body parts include:
- Extensive root systems: The root anchors the plant and absorbs minerals and water from the soil.
- Stem: The stem supports the growing body of the plant.
- Leaves: For most flowering plants, the leaves are the main sites of photosynthesis.
These plants dominate the surface of the earth as well as the vegetation in more environments than any other group of plants. They dominate terrestrial habitats in particular. Thus, they are the most important common source of food for birds and mammals, as well as humans. Additionally, these flowering plants are the most economically important group of green plants. They serve as a source of timber, pharmaceuticals, ornamentals, fiber products, and other commercial products.
Together with gymnosperms, angiosperms are vascular plants. Vascular plants are those plants that possess specialized vascular tissues. The phloem and xylem are the two types of vascular tissue. They are responsible for transporting minerals, water, and photosynthetic products within the plant.
Vascular plants, contrary to non-vascular plants, can grow much larger. Within the plant, the xylem and phloem provide a means of transporting water to great heights. This allows the vascular plant to grow upward towards the sun.
The structure of vascular plants is much different from the structure of non-vascular plants. There is no or little differentiation between the different cells of a non-vascular plant. Whereas, the specialized vascular tissues in vascular plants are arranged in distinct patterns. These patterns are arranged based on the division and species that the vascular plant belongs to.
The function of Xylem in vascular plants
The xylem specializes in the transportation of water and minerals from the roots of the plants to the leaves. Xylem is a vascular tissue that is made mostly of the structural protein lignin and dead cells. Angiosperms as vascular plants achieve the transportation of water by creating pressure on the water on multiple fronts.
In the roots of the plant, the water is absorbed into the tissues of the plant. Into the xylem, the water flows creating upward pressure. Then, the water is used at the leaves of the plants and evaporates out of the stoma of the leaves. The stomata are small pores on the leaves that transpire water. They pull upward on the column of water in the xylem and the water moves upward through the actions of adhesion and cohesion.
Photosynthesis takes place in the leaves of these plants. Angiosperms as vascular plants use the process of photosynthesis to extract energy from the sun. They then, store it in the form of sugar which is modified into other forms and transported to other parts of the plant like the root and stem that cannot photosynthesize.
The function of phloem in vascular plants
The phloem of vascular plants is designed especially for the transportation of sugar. The phloem is different from the xylem and it is made of partially living cells. It aids the transportation of sugars through transport proteins in the cell membranes. Also, the phloem is connected to the xylem and can add water to help in the dilution and movement of sugar. The phloem when commercially harvested is known as syrup or sap e.g Maple syrup.
- All angiospermous plants have flowers at some stage in their life. These flowers are the reproductive organs of the plant that provides them with a means of exchanging genetic information.
- The sporophyte of these plants is evolved into leaves, stems, and roots.
- Their vascular system possesses true vessels in the xylem and has companion cells in the phloem.
- In these plants, the flower is a structure formed from the arrangement of the carpels (megasporophyll) and the stamens (microsporophyll).
- There are four microsporangia in each microsporophyll of these plants.
- Angiosperms have their ovules enclosed in the ovary at the base of the megasporophyll.
- These plants are heterosporous. This means they produce two kinds of spores which are microspores (pollen grains) and megaspores.
- In flowering plants, reproduction occurs by pollination, where pollen grains are transferred from the anther to the stigma. This process of pollination is responsible for the transfer of genetic information from one flower to the other.
- The pollen grains of these plants, in contrast with non-flowering plants, are much smaller than the non-flowering plants’ gametophytes or reproductive cells.
- Angiospermous plants’ sporophytes are diploid.
- Their root system is very complex and includes the cortex, phloem, xylem, and epidermis.
- The flowers of angiospermous plants undergo double and triple fusion, thereby leading to the formation of a diploid zygote as well as a triploid endosperm.
- These plants can survive in a wide variety of habitats, even marine habitats.
- In angiosperms, the process of fertilization is quicker. Also, as a result of the smaller female reproductive parts, their seeds are produced quickly.
- All flowering plants have stamens that serve as the reproductive structures of their flowers.
- Their carpels enclose developing seeds that likely develop into a fruit.
- The production of the endosperm is unique to angiosperms. After fertilization, the endosperm is formed as a source of food for the developing seed and seedling.
The structural features of angiosperms (flowering plants)
The body of an angiosperm is made up of three parts: roots, stems, and leaves. These basic organs make up the vegetative (non-reproductive) body of the plant. The shoot of the plant is made up of the stem with the leaves attached. Whereas, the roots make up the root system. Let’s look at each feature of these plants in detail.
The roots of the plant anchor it to the ground. They absorb minerals and water as well as provide a storage area for food. Basically, the root systems in angiosperms are of two types. These include:
- Primary root system
- Adventitious root system
Primary root system
The primary root system is the common type of root. It consists of a taproot that grows vertically downwards exhibiting positive geotropism (see Tropism). Smaller lateral roots (secondary roots) are produced from the taproot. These secondary roots grow horizontally or diagonally, producing their own smaller lateral roots (tertiary roots). Therefore, produced from the taproot, are several roots of descending sizes.
The majority of eudicotyledons, for instance, the dandelion, produce taproots. The primary (taproot) system, in some cases, is modified into a fibrous or diffuse system. In such cases, the initial secondary roots are exact in size or exceed the primary root. Hence, there are several large positively geotropic roots that generate higher-order roots. These higher-order roots may likely grow to the same size. Thereby, there is no well-defined single taproot in the fibrous root system. Generally, the fibrous root systems are more shallow than taproot systems.
Furthermore, some lateral roots of mangroves in saline mud flats become specialized as pneumatophores. These pneumatophores are lateral roots that grow upward exhibiting negative geotropism for varying distances. Also, they function as the location of oxygen intake for the immersed taproot system. However, there are several examples of root diversity in angiosperms. Such root diversity is rare in any other group of vascular plants.
Adventitious root system
The adventitious root system is the second type of root system. This root system differs from the primary root system. The difference is that the taproot or primary root is usually short-lived and is replaced by many roots forming from the stem. The majority of monocotyledons have these adventitious root systems. Such examples are orchids, bromeliads, and many other tropical epiphytic plants. Many other monocotyledons and grasses give rise to fibrous root systems with the development of adventitious roots.
However, when these adventitious roots are modified for aerial support, they are called prop roots like in corn or some figs. The large woody prop roots in several tropical rainforest trees develop from adventitious roots on horizontal branches. Hence, they provide extra anchorage and support. Also, several bulbous plants have adventitious roots that are contractile. These roots haul the bulb deeper into the ground as it grows. Moreso, climbing plants with the help of specialized adventitious roots, usually grip their supports.
Conclusively, for specific functions, several tap root and adventitious root systems have become modified. The most common modification is the formation of tuberous (fleshy) roots to store food. For instance, the tuberous roots, beets, and carrots are from taproots. Whereas, cassava is a tuberous root modified from an adventitious root.
Stems of flowering plants
The stem of the plant is the aerial axis of the plant. It bears the leaves and flowers of the plant. Also, it transmits water and minerals from the roots as well as food from the site of synthesis to where it is utilized. Through a transition region referred to as the hypocotyl, the plant’s main stem is continuous with the root system. This hypocotyl in the developing embryo is the embryonic axis that bears the seedling leaves called the cotyledons.
The node on a maturing stem is the area that a leaf attaches to the stem. Between successive nodes is a region called an internode. At the nodes, stems bear leafy shoots (branches) that arise from buds (dormant shoots). Different plants form as a result of simply changing the lengths of the internodes. For instance, rosette plants like lettuce result from the extreme shortening of the internodes. In such a case, the leaves develop, but the internodes between the leaves don’t elongate until the plant bolts when flowering. Also, twining vines such as yam result from the extreme lengthening of the internodes.
Lateral branches develop from axillary or lateral buds. They develop at the angle between the stem and the leaf. Also, these lateral branches can develop from terminal buds at the end of the shoot. In tropical plants, these buds have a short or non-existent period of dormancy. Whereas, in temperate-climate plants, the periods of dormancy of these buds are extended.
Types of branching in angiosperms
To understand the diversity of the shoot system in angiosperm, the precise positional relationship of the stem, leaf, and axillary bud is crucial. Understanding this relationship makes it easier to identify leaves that are highly modified that no longer look like leaves, or probably highly modified stems that resemble leaves. Furthermore, there are two forms of branching in angiosperms. These include:
- Dichotomous branching
- Axillary branching
In dichotomous branching, the branches form due to an equal division of a terminal bud. This means a bud is formed at the apex of a stem. The terminal bud divides into two equal branches that are not derived from axillary buds, even though the axillary buds are present elsewhere on the plant body. Among angiosperms, there are very few examples of dichotomous branching. This type of branching is only found in some palms, cacti, palms, and bird-of-paradise plants.
In angiosperms, there are two modes of axillary branching. They include monopodial branching and sympodial branching.
The monopodial branching happens when the terminal bud continues to grow as a central leader shoot while the lateral branches remain subordinate. An example is beech trees. The overall shape of plants with monopodial growth is usually pyramidal.
Sympodial branching, on the other hand, happens when the terminal bud ceases to grow and an axillary bud becomes the new leader shoot. An example is the Joshua tree. The terminal bud may cease to grow, usually because a terminal flower has formed and the new leader shoots which happen to be the auxiliary buds are called renewal shoots. The plants with sympodial growth usually resemble candelabra.
Furthermore, many different tree structures have evolved by combining monopodial and sympodial branching in one plant. A typical example is seen in dogwoods where the lateral branches are sympodial and the main axis is monopodial.
The leaf of flowering plants is composed of the following:
- A leaf base: This is the slightly expanded area that attaches the leaf to the stem.
- Two stipules: When the two (paired) stipules are present, they are located on each side of the leaf base. They may resemble a leaflike structure, spines, scales, or glands. Stipules, however, forms at the base of the leaf protecting the developing blade.
- A petiole: This is a stalk. It connects the leaf base with the blade.
- The blade (lamina): This is the main photosynthetic surface of the plant. It is green in color and is flattened. In a plane perpendicular to the stem, the blade is flattened.
There are three patterns of how leaves are arranged on stems in angiosperms. They include:
- Alternate pattern: In this arrangement, the leaves of the plant are single at each node in alternate-leaved plants. In an ascending spiral, the leaves are borne alternately along the stem.
- Opposite (paired) pattern: The leaves are paired at a node in opposite-leaved plants. These leaves are borne and arranged opposite to each other.
- Whorled pattern: Plants with whorled leaves have 3 or more equally spaced leaves at a node.
A leaf is said to be simple when only a single blade is inserted directly on the petiole. They may be lobed variously along their margins. Their margins, however, can be entire and smooth or these leaves may be lobed in several ways. The leaf margin is the perimeter of the leaf that lies between the base and apex of the leaf. These leaf margins are of various types: Entire, serrate, crenate, dentate, lobed, sinuate.
At right angles, the coarse teeth of dentate margins stick out, whereas the teeth of serrate margins project toward the leaf apex. Then, the crenulate margins are rounded. These leaf margins of simple leaves may be lobed in either of two patterns. Pinnate and palmate are the two patterns at which the leaf margins may be lobed.
The leaf blade (lamina) in pinnately lobed margins is indented equally deep along each side of the midrib. An example is seen in the white oak. Whereas, in palmately lobed margins, the lamina is indented along several major veins. An example is red maple. Also, a great variety of base and apex shapes are found in leaves. Several leaves have only some of the leaf parts. For instance, in several leaves, the petiole is absent and thus attaches directly to the stem and some leaves lack the stipules. The leaf of the willow is a typical example of simple leaves.
There are two or more subunits in a blade of compound leaves. These subunits are called leaflets. In the palmately compound leaf, the leaflets diverge from a single point at the distal end of the petiole. Whereas, in pinnately compound leaves, there is a row of leaflets formed on either side of an extension of the petiole. This is called the rachis.
An amount of pinnately compound leaves branch again as they develop a second set of pinnately compound leaflets called bipinnately compound leaves. The several degrees of compoundness in leaves like bipinnately or tripinnately compound leaves cause these leaves to usually appear as shoot systems. However, it is possible to distinguish them. Usually, axillary buds are found in the angle between the petiole and the stem of pinnately or palmately compound leaves. Thus, the axillary buds are not in the axils of leaflets. For instance, the leaf of the walnut is pinnately compound (feather-shaped). Whereas, the horse chestnut leaf is palmately compound (hand-shaped).
Modification of Leaves
For special functions, the leaves of angiosperms are usually modified. Such specific functions include storage, climbing and substrate attachment, tapping, digesting insect prey, protection against predation or climatic conditions.
Leaves are protective bud scales in temperate trees. The shoot growth is resumed in the spring and from bud scales they usually display a complete growth series to fully developed leaves. However, before the rest of the leaf, the stipules often develop and protect the young blade. Then, when the leaf matures, they often shed.
Spines as modified leaves
The spines on plants are modified leaves and the spines in cacti are wholly transformed leaves. They protect the plant from herbivorous animals. During the day, they diverge heat from the stem, and during the cooler night, they collect and drip condensed water vapor. The stipules in many species of the spurge family plants are modified into paired stipular spines. In ocotillo, the leaf blade develops fully and falls off while the petiole remains as a spine. The ocotillo is also called Jacob’s staff, coachwhip, or vine cactus.
Modified succulent leaves for storage
For water storage, many desert plants like stone plants and aloe vera develop succulent leaves. The succulent leaf bases of underground bulbs, for example, crocus and tulip, are the most common form of storage leaves. They serve as organs for water storage or food storage or even both. Moreso, through specialized hairs on the leaf surface, some plants absorb water. Many of the nonparasitic plants that grow on the surfaces of other plants (epiphytes) are such plants that possess specialized hairs on their leaf surface. A typical example is the bromeliads. Also, the swollen petioles in the water hyacinth is a leaf modification that keeps the plant afloat.
Tendrils and hooks as modified leaves for support
The modification of leaves or leaf parts can provide support. For instance, the tendrils and hooks are the most common modifications for such support. The leaf tip of the blade in the flame lily elongates into a tendril. Hence, twining around other plants for support. Also, the terminal leaflet of the compound leaf in the garden pea develops as a tendril and the petioles in the nasturtium and Clematis coil surround other plants for support. Likewise, the stipules in catbrier serve as tendrils.
Furthermore, some of the leaflets in certain vining angiosperms with compound leaves have been modified into grapnel-like hooks. A typical example is Tecoma radicans. Like in bananas, many monocotyledons have sheathing leaf bases that are coaxially arranged to form a pseudotrunk. The pseudotrunk in many epiphytic bromeliads serves as a water reservoir.
Leaf modification for predation
Another kind of leaf modification is seen in carnivorous plants. They use their leaves to attract and trap insects. The leaves have glands that secrete enzymes. These enzymes digest the captured and trapped insects. Then, the leaves absorb the nitrogenous compounds (amino acids) and other products of digestion. These plants that get their nitrogen source from insects tend to grow in nitrogen-deficient soils.
Shoot system modifications
The entire shoot system is usually modified for special functions. Such special functions include protection, climbing, adaptation to arid habitats, water storage, or food storage. Generally, the modifications involve changes to the structure and shape of the stem as well as the reduction of the leaves to small scales.
Tendrils and hooks
Auxillary buds in the grape and the passionflower develop as tendrils with reduced leaves and suppressed axillary buds. The axillary tendrils in the grape are actually modified and reduce inflorescences. In the plant (Strychnos nux-vomica) from which strychnine is obtained, the axillary buds develop into hooks for climbing. The English ivy has tendrils that produce enlarged cuplike holdfasts.
Also, thorns portray the modification of an axillary shoot system. It is a modification where the leaves of the plants are reduced and die quickly. The stems are intensively sclerified and grow for only a limited time. These thorns tend to protect the plant from herbivores. Typical examples of such protection are seen in the honey locust, anchor plant, and Citrus.
Also, we have cladodes (also referred to as phylloclades or cladophylls) which are shoot systems that do not develop leaves. Instead, the stems become flattened and take charge of the photosynthetic functions of the plant. The cladode is a green flat leaflike structure. The true leaves in asparagus are the scales found on the asparagus spears and the structure called cladodes develops in the axils of the scale leaves, once the thick fleshy asparagus spears continue to grow.
Cladodes are present in all cacti and many desert members of the spurge and milkweed families possess similar vegetative morphologies. These morphologies are derived by modifying different parts to look and function in the same way. However, each of these plant groups has reduced leaves, columnar, water-storing green stems, as well as protective spines or thorns. Oftentimes, they are called stem succulents.
The leaves on the main stems in the cacti last for a very short time. They do not actually develop as scale leaves and the leaves of the axillary buds in the cacti develop as spines. The cacti are adapted to dry or arid habitats. In Asia and Africa, the Euphorbiaceae and Apocynaceae occur in similar habitats. In the Cactaceae, the leaves reduction is so severe. As a result, the epiphytic cacti of the Neotropics can no longer produce leaves. Instead, they produce cladodes that superficially resemble leaves. These cladodes are thin and flat.
Rhizomes, Tubers, and Corms
Several shoot systems have been modified into food storage organs or reproduction organs, or even both. These organs are called rhizomes, tubers, and corms. Rhizomes are distinct from roots in that they have nodes with reduced leaves and internodes. They are horizontal subterranean shoots possessing scale leaves and adventitious roots on the underside. Their main functions are the storage of food and vegetative reproduction. The food stored in the rhizomes allows the plants to survive periods of extended winter and drought. Lots of rhizomes are perennial. From the nodes, they send up new shoots spreading the colony.
Usually, the rhizome’s terminal bud becomes upright. It then flowers and a rhizome axillary bud becomes a renewal shoot. Several economically important plants like banana, and virtually all grasses, as well as bamboo, and sugarcane, have rhizomes. These plants are propagated mainly by fragmentation of the rhizome.
The growing tips of rhizomes in some plants become much-enlarged food storage organs. These storage organs are called tubers. A typical example is the common potatoes that form such tubers. The reduced scale leaves as well as their associated axillary buds form the eyes of the potato.
Tubers should not be interchanged with tuberous roots. It is key to note that tuberous roots are modified roots, while tubers are modified shoots. Their similarity, however, is derived from the fleshy nature of both organs. Tubers and tuberous roots are involved in water and food storage, though only tubers function in vegetative (non-sexual) reproduction. Tuberous roots develop from taproots and adventitious roots as in carrots and dahlias, respectively.
The corm is another distinctive modification for food storage. It is a short upright shoot system. The corm has a thick hard stem that is covered with thin membranous scale leaves. Jack-in-the-pulpit and gladiolus have corms. These corms are usually fibrous and hard and fibrous functioning for overwintering and resistance to drought.
Stolon or Runners
Runners or stolons are the slender creeping stems that grow above the soil surface. They have scale leaves and thus develop roots. Therefore, new plants are formed either terminally or at a node. For instance, the stolons in strawberries are used for propagation. The buds develop at nodes along the stolons and grow into new strawberry plants.
Examples of angiosperms
- Grains such as corn, rice, wheat, oats, barley, sorghum, and rye, etc
- Trees like Magnolia, palm tree, oak, maple, etc.
- Parsley family include plants like cow parsley, sea holly, parsnip, ajwain, angelica, lovage, anise, fennel, asafoetida, dill, caraway, cumin, carrot, coriander, celery, chervil, silphium, giant hogweed, fool’s parsley, poison hemlock, water hemlock, spotted cowbane, and some species of water dropwort.
- Flowers like buttercup, roses, daisy, lilies, sunflowers, Bellflower, poppies, etc.
- Heaths such as blueberry, cranberry, huckleberry, rhododendron, Erica, Cassiope, Daboecia, and Calluna.
- Timber-yielding plants such as teak, mahogany, etc.
- Fruits like jackfruit, banana, apple, guava, jackfruit, etc.
- Medicinal plants such as Atropa, rauwolfia, Cinchona, etc.
- Vegetables like tomatoes, cabbage, onions, garlic, broccoli, eggplants, spinach, potatoes, etc.
The above are few examples of flowering plants.
Fertilization and embryogenesis
Double fertilization occurs in angiosperms. This refers to a process in which two sperm cells fertilize cells in the ovule of the angiospermous plant. Fertilization begins when a pollen grain adheres to the stigma of the pistil which is the female reproductive structure. It then germinates and develops a long pollen tube.
As this pollen tube is developing, a haploid generative cell moves down the tube behind the tube nucleus. By mitosis, the generative cell divides to give rise to two haploid sperm cells. However, the pollen tube makes its way from the stigma as it grows. It travels from the stigma, down the style, and into the ovary.
It is in the ovary that the pollen tube reaches the micropyle of the ovule. Then, it finds its way into one of the synergids, thus, releasing its sperm cells. As the sperm cells are released into the synergid, the synergid degenerates. Then, one of the sperm cells finds its way to fertilize the egg cell. Thereby, producing a diploid (2n) zygote. Whereas, the second sperm cell fuses with both central cell nuclei. Thereby, producing a triploid (3n) cell.
As the triploid cell develops into an endosperm, the zygote develops into an embryo. The endosperm, however, serves as the embryo’s food supply. Then, the ovary develops into a fruit of the angiosperm plant, while the ovule develops into a seed. The embryo and endosperm progressively develop within the embryo sac. As that occurs, the sac wall enlarges. It then combines with the integument and the nucellus to form the seed coat. Then, the ovary wall develops to form the pericarp or fruit.
The features of the seed coat have a definite relation to that of the fruit. They help in dissemination and protect the embryo. Also, they may directly promote germination. Generally, the indehiscent fruit in flowering plants gives protection to the embryo as well as securing dissemination. The seed coat in this case is only slightly developed. Whereas, the seedcoat is well developed if the fruit is dehiscent and the seed is exposed. Thereby, it must carry out the functions that are otherwise executed by the fruit.
Some plant species like the Asteraceae family have evolved to fruit morphs or exhibit heterocarpy. The fruit morphs are produced from one plant and vary in size and shape. This influences the range of dispersal as well as germination rate. These fruit morphs have high chances for survival as they are adapted to different environments.
Meiosis in flowering plants
Angiosperms generate gametes through a specialized cell division process called meiosis. Meiosis, however, occurs in the ovule. To produce four cells, a diploid cell (megaspore mother cell) undergoes meiosis in the ovule. Therefore, as a result of this meiosis, four cells called megaspores with haploid nuclei are produced.
One out of these four megaspores produced then undergoes three successive mitotic divisions. Thus, an immature embryo sac referred to as megagametophyte is produced with eight haploid nuclei. Then, these nuclei via cytokinesis are segregated into separate cells to generate 3 antipodal cells, 2 synergid cells, and an egg cell. However, in the central cell of the embryo sac, two polar nuclei are left there.
In the male anther (microsporangium), through meiosis, pollen is also produced. A diploid microspore mother cell during meiosis undergoes two successive meiotic divisions to generate 4 haploid cells called microspores or male gametes. However, after further mitoses, each of these microspores becomes a pollen grain that contains two haploid generative (sperm) cells as well as a tube nucleus. The pollen grain forms a pollen tube when it makes contact with the female stigma. This pollen tube develops down the style and then enters into the ovary.
Therefore, a male sperm nucleus in the process of fertilization fuses with the female egg nucleus. Thereby, producing a diploid zygote that further develops into an embryo within the newly forming seed. Hence, as the seed germinates, a new plant can grow and mature. During meiosis in a diploid cell, the main process is the exchange of genetic information which involves the pairing of homologous chromosomes and homologous recombination between homologous chromosomes. This event enhances the production of increased genetic diversity among progeny. Also, it promotes the recombinational repair of damages in the DNA which is to be passed on to progeny. However, some authors emphasize diversity to explain the adaptive function of meiosis in flowering plants, whereas, others emphasize DNA repair.
Apomixis (asexual reproduction in flowering plants)
Reproduction through asexually formed seeds is apomixis. This is found naturally in about 2.2% of angiospermous plant genera. Gametophytic apomixis is a type of apomixis seen in a dandelion species. In this type of apomixis, an unreduced embryo sac is formed as a result of incomplete meiosis. Also, an embryo develops without fertilization from an unreduced egg inside the embryo sac. Moreso, some angiospermous plants through a type of apomixis referred to as nucellar embryony are able to produce fruits. Such examples include several citrus varieties.
The life cycle of angiosperms
- Angiosperms carry out double fertilization. They are seed-producing plants and generate male and female gametophytes. Angiosperms generate microspores that develop into pollen grains which are the male gametophytes.
- The megaspores, then form an ovule that contains the female gametophytes.
- Then, the megasporocyte undergoes meiosis in the ovule. Thereby, 4 megaspores are generated where 3 are small and 1 is large.
- However, only the large megaspore survives, thus producing the female gametophyte (embryo sac).
- The pollen grain extends its pollen tube as it reaches the stigma to enter the ovule and deposits two sperm cells in the embryo sac.
- These 2 available sperm cells enable the occurrence of double fertilization. Thus, resulting in a diploid zygote and a triploid cell. The diploid zygote is the future embryo, whereas the triploid cell is the future endosperm that acts as a food store.
- Angiosperm species could be hermaphroditic, monoecious, or dioecious. Hermaphroditic species have their stamens and pistils contained on a single flower whereas; monoecious species have their stamens and pistils contained on separate flowers but on the same plant. Then, dioecious species have staminate and pistillate flowers occur on separate plants.
The major phase of the life cycle of an angiosperm is the adult or sporophyte phase. Angiosperms are heterosporous, thereby generates microspores. These microspores produce pollen grains which are the male gametophytes. Then, the megaspores form an ovule that contains the female gametophytes.
In the microsporangia of the anther, by meiosis, the male gametophytes divide to generate haploid microspores. These haploid microspores later undergo mitosis and develop into pollen grains. In each pollen grain, there are two cells contained. One generative cell divides into two sperm and the second cell becomes the pollen tube cell.
The ovary of the carpel encloses the ovule which contains the megasporangium. This megasporangium is protected by two layers of integuments including the ovary wall. A megasporocyte undergoes meiosis within each megasporangium. Thus, generating 4 megaspores: three of the megaspores are small, whereas one is large. However, only the large megaspore survives, thus producing the female gametophyte (embryo sac).
To form an eight-cell stage, the megaspore divides three times. Two of these cells move to the equator and fuse to form a 2n polar nucleus. Then, four of the cells move to each pole of the embryo sac. Three of the cells that are away from the egg form antipodals whereas the two cells that are closest to the egg become the synergids (helper cells).
Therefore, the mature embryo sac contains 1 egg cell, 2 synergids, 3 antipodal cells, and 2 polar nuclei in a central cell. Hence, when a pollen grain gets to the stigma, a pollen tube extends from the grain. The pollen tube grows down the style and enters through the micropyle. This micropyle is an opening in the integuments of the ovule. Then, two sperm cells are deposited in the embryo sac.
As a result, double fertilization occurs. A diploid zygote (future embryo) is formed as one sperm and the egg combine. Then, the other sperm fuses with the 2n polar nuclei. Thereby, forming a triploid cell. The triploid cell develops into an endosperm. However, the endosperm is a tissue that serves as a food reserve.
This zygote then develops into an embryo. The embryo. however, possesses a radicle, or small root, and 1 (monocot ) or 2 (dicot) leaf-like organs. These leaf-like organs are called cotyledons. However, what distinguishes the two major groups of angiosperms is the difference in their number of embryonic leaves. They are grouped into monocots and eudicots.
Seed food reserves are stored in the form of complex carbohydrates, lipids, or proteins outside the embryo. The cotyledons act as channels to transmit the broken-down food reserves. They transmit this food from where it is stored inside the seed to the developing embryo. However, the seed is made up of the endosperm with food reserves, a toughened layer of integuments that form the coat, and the well-protected embryo at the center.
However, angiospermous plant species could be hermaphroditic, monoecious, or dioecious. Hermaphroditic species have their stamens and pistils contained on a single flower whereas; monoecious species have their stamens and pistils contained on separate flowers but on the same plant. Then, dioecious species have staminate and pistillate flowers occurring on separate plants. Moreso, anatomical and environmental barriers enhance cross-pollination which is brought about by an animal such as a bird or insect, or physical agents like water or wind. However, genetic diversity in species increases due to cross-pollination.
Classification of Angiosperms
Angiosperms, however, with a few exceptions can be divided into monocotyledons (monocots) and dicotyledons (dicots). The cotyledons of the plant provide a useful way of classifying plants. They are the parts of the seeds that develop to form leaves.
- Their seeds have a single cotyledon in the embryo.
- Monocots possess pollen with a pore or single furrow.
- Their leaves are simples and the leaf veins are parallel to each other.
- Monocots have a network of roots. They contain adventitious roots.
- The flower parts of a monocot are in multiples of three. i.e Each floral whorl has three members.
- These plant groups have a scattered vascular tissue system.
- Monocots have closed vascular bundles and are large in number.
- Examples of monocotyledons include orchids, bananas, grasses, sugarcane, lilies, etc.
- The seeds of dicots have two cotyledons.
- Dicots have pollen with 3 pores or furrows.
- These plants usually have a woody stem and secondary growth.
- Dicots contain tap roots rather than adventitious roots.
- The leaves of dicots depict a reticulate venation where they possess net-like leaf veins.
- The flower parts of dicots are tetramerous or pentamerous, i.e their flower parts come in multiples of four or five.
- The vascular system of dicots is organized in rings.
- Examples of dicotyledons include roses, grapes, peas, sunflower, daisies, tomatoes, etc.
Economic Importance of Angiosperms
They contribute to the food chain
Angiosperms, in terms of biomass and individual numbers, are the most numerous elements of the terrestrial ecosystem. These plants serve as an important source of food for animals and other living organisms. Their organic compounds (principally carbohydrates) serve as the only source of energy for most heterotrophic organisms. These plants themselves use the organic compounds for fueling their basic metabolisms as well as synthesizing cellular structures.
Heterotrophic organisms need an organic source of carbon that originated as a component of another living organism. Whereas, autotrophic organisms only need an inorganic source of carbon (CO2). The photosynthetic pigments in the plant cells trap solar energy, converting it into chemical energy. This energy is then stored in the plant tissues. Hence, this trapped energy is transferred from one organism to another in a food chain.
The herbivores that consume the angiosperms are in turn eaten by carnivores and so on transferring the trapped energy. Many thousands of animals are supported by a single angiosperm tree in a temperate forest. This relationship entails the basic importance of angiosperms to the ecological food web.
The body of an angiosperm contributes in many ways to the food chain. Its vegetative parts such as the leaves and stem supports and are eaten by plant-eating animals. A large number of insects and invertebrates during all phases of their life cycle rely on the shoots of the plants for food. Also, the reproductive organs of the plants such as the seeds, flowers, and fruits serve as a source of energy for many animals.
Flowers of angiosperms as a source of food
The pollen grains of angiosperms are important to many pollinating insects, especially bees and their flowers provide food. Its floral nectar secrets amino acids and sugars. These flowers usually release fragrances that attract pollinators that feed on the nectar. Nectar-feeding animals range from insects such as bees, moths, mosquitoes, and flies, to birds such as hummingbirds, sunbirds, and honeyeaters. Also, there are nectar-feeding mammals such as small rodents, bats, and small marsupials.
There are also nectaries on the nonfloral, or vegetative, parts of some angiosperms. Such parts include the leaves and the petioles of the bull’s horn thorn. The hollow modified spinous structures of bull’s-horn thorn house ants that feed on the nectar. There is a mutual relationship between these two organisms as the ants in return protects the acacia plant. These ants, in return for the nectar they gain from the plant, destroy and attack animals of various sizes that come in contact with the acacia plants. Also, they attack other plants too that contact the acacia plant. As a result of this mutualistic relationship, the bull’s-horn thorn is protected by the ants. It is protected from herbivorous animals and other plants that compete with the plant for the available minerals, space, and light.
Fruits as a food source
The fruits angiosperms produce serve as a primary food source for several birds, bats, mammals, and even fish. Also, the seeds of the fruits are important food for various animals, especially birds and small rodents. All these animals consume the fruits and seeds of the angiosperm. They carry them to new habitats, where the flowering plants propagate.
Angiospermous plants that are cultivated for their fruits are seen in tropical, temperate, or subtropical regions. The plants in the temperate regions are generally deciduous. For their growth, they tolerate or require a cool period. Pears and apples are important pome fruits of the family Rosaceae. Peaches, plum, nectarines, and cherries are some well-known fruits of the family. There are other temperate fruits that grow on low plants, bushes, or vines. They include grapes, blueberries, cranberries, and strawberries.
Tropical fruits can only tolerate and survive temperatures above freezing. They tend to be grown on evergreen plants. However, subtropical plants are either deciduous or tropical. They are not susceptible to temperatures slightly below freezing. Tropical and subtropical plants include citrus, figs, avocados, dates, bananas, olives, papayas, and pineapples.
There are many plants that are classified popularly as vegetables which are in actuality fruits. This is because they develop from the reproductive structures of the plant. The common type of such plants include pumpkins, squashes, and gourds. The cucumber too from a branching vine produces a fruit. Okra produces small fruit pods, It is a warm-weather crop. Breadfruit is a staple crop that is native to the Pacific Islands. It provides a rich source of starch and calcium.
Secondary compounds from angiosperms
Angiospermous plants produce secondary compounds that are of importance to some organisms. Alkaloids, essential oils, quinones, and glycosides are examples of such secondary compounds. These flowering plants have evolved a comprehensive array of unpalatable or toxic secondary plant compounds. The compounds protect them from foraging herbivorous organisms. However, some insects store these secondary compounds successfully in their tissues and make use of them as a protective mechanism against predation.
Though some angiospermous plants for protection produce secondary compounds that are toxic, some, however, produce non-toxic secondary compounds. In fact, many of these non-toxic secondary compounds are found in herbs and spices. A typical example is the cloves and the dried flower buds of Syzygium aromaticum. For a long time, herbs and spices have been used in cooking.
Usually, herbs are young shoots or leaves of non-woody plants. Also, a few leaves from woody plants and bay leaves are considered herbs. Spices are gotten from the bark, roots, seeds, fruits, rhizomes, and flower parts of the plant. They are the highly flavored aromatic parts of plants. Spices are high in essential oil content. Also, several beverages are derived from angiosperms. Such examples are coffee, tea, some soft drinks, and many alcoholic beverages. Coffee is gotten from the plant Coffea Arabica, whereas tea is derived from Camellia sinensis. Alcoholic beverages like wine are gotten from grapes and beer and whiskey from cereal grains.
The angiosperm flora determines several habitat features
Angiosperms are the primary component of the terrestrial biosphere. As a result, their flora determines many features of the habitat. Some of their flora as discussed are available food sources. Some form the parts of the forest canopy, and grazing land. They serve as nesting sites and supply materials for many mammals and birds.
The flora of angiosperms is the principal living space for many reptiles, primates, and amphibians. Animals such as frogs, salamanders, as well as many aquatic insects and larvae inhabit the tank bromeliad that traps water in its crowns. However, these animals that inhabit the water-filled insectivorous pitcher plant leaves have adapted to the digestive fluids of the leaves that causes a hostile environment.
Angiosperms contribute to biodiversity and habitat. Thus, they are so extremely important that animals, as well as human life, are totally dependent on them. They are important in the habitat. As a result, a significant loss of angiosperms would reduce the variety of food sources. Also, the oxygen supply in the habitat would reduce. Thereby, drastically altering the amount and distribution of the world’s precipitation. Doubtlessly, there are many food sources and sources of medicine that are yet to be discovered in this group of vascular plants.
Angiosperms are used by humans for various products
Angiospermous plants are also important to humans as they are important to other animals. They serve as the major source of food for humans. This food source is either by directly consuming the plants or the indirect consumption through the consumption of herbivores. Flowering plants are a primary source of consumer goods like building materials, spices, textile fibers, herbs, and pharmaceuticals.
Valuable pharmaceuticals are derived from angiosperms. Virtually all medicinal except antibiotics are derived from angiosperms. Directly, they can be derived from compounds produced by angiosperms or, if synthesized, were originally discovered in angiosperms. Such pharmaceuticals are vitamins, aspirin, narcotics, and quinine. Examples are the vitamin C that is originally extracted from fruits. Aspirin is derived from the bark of willows and narcotics like opium and its derivatives are gotten from the opium poppy, Papaver somniferum. The quinine is derived from Cinchona bark.
Additionally, some toxic angiosperm compounds have proven to be effective in the treatment of some diseases. Such disease examples include certain forms of cancer (such as acute leukemia), and heart problems. Moreso, muscle relaxants are derived from curare and are used during open-heart surgery.
In fact, many of the non-toxic secondary compounds produced by angiosperms are found in herbs and spices. A typical example is the cloves and the dried flower buds of Syzygium aromaticum. For a long time, herbs and spices have been used in cooking. Usually, herbs are the young shoots or leaves of non-woody plants.
Also, a few leaves from woody plants and bay leaves are considered to be herbs. Spices are gotten from the bark, roots, seeds, fruits, rhizomes, and flower parts of the plant. They are the highly flavored aromatic parts of plants. Spices are high in essential oil content. Also, several beverages are derived from angiosperms. Such examples are coffee, tea, some soft drinks, and many alcoholic beverages. Coffee is gotten from the plant Coffea arabica, whereas tea is derived from Camellia sinensis. Alcoholic beverages like wine are gotten from grapes and beer and whiskey from cereal grains.
Angiosperm as agricultural food plants
Globally, the most common and important food plants include:
- Cereals from the grass family
- Potato family that include tomatoes, potatoes, eggplant, and chili peppers
- Legumes or beans (Fabaceae)
- Squash family that include melons, pumpkins, and gourds
- Mustard family that includes cabbage, radish, broccoli, cauliflower, and other vegetables
- Rose family that include apricots, almonds, peaches, cherries, pears, apples, loquats, peaches, raspberries, and strawberries
On a local level, members of many angiosperm families are used for food. An example is the Ullucu and cassava. Throughout the world and in the tropics, tropical angiosperm trees are an important source of timber.
The angiosperm has a number of uses as food and is cultivated for that purpose. Examples include grains, fruits, sugars, oils, vegetables, nuts, and spices. Also, these plants and their products serve many other needs, like dyes, timber, fibers, medicine, fuel, and ornamentals. Some angiospermous plants even serve more than one function. For instance, the seeds of the kapok fruit produce a water-repellent fiber. This fiber is used in thermal and sound insulation. Moreso the seed of the kapok fruit yields an edible oil that is used in cooking, soap making, and lubricants. However, the oil cake is high in protein, thus, it is fed to livestock. Then, the soft light wood is used to make boats and furniture.
Angiosperms convert solar energy into starch. Starch is the energy-rich storage form of sugar. These plants reserve the starch in the endosperm of the seed for when the seedling germinates and grows. Throughout the world, all members of the grass family as well as corn, rice, wheat, oats, barley, sorghum, and rye are among the most economically important grains. However, sugar beet and sugarcane are rich sources of natural sugar.
Corn and its derivative, for instance, is a food source for humans and domesticated animals. Examples of corn derivatives include cornstarch and corn oil which are used in making cosmetics, paints, adhesives, soaps, varnishes, and linoleum. In the United States, dent corn amongst the many cultivars of corn is a widely used feed type. Also, rye, wheat, and barley are all members of the same tribe- Triticeae which is within the grass family. Wheat is one of the oldest cultivated food crops. Barley is consumed by humans and is also used for malting and livestock feed. Usually, rye is used as livestock feed. However, rye can be used in distilling liquor and baking. Globally, rice is one of the main cereal crops. It is a semiaquatic annual grass.
Vegetables have diverse nutritional content. They constitute perhaps the greatest diversity of form. However, vegetables are grown for one or more of their parts. They can be grown for their leaves, flowers, shoots, or their underground parts, like tuberous roots, bulbs, corms, tubers, and rhizomes. For instance, Asparagus is a perennial plant that is grown for its succulent green cladodes. The green cladodes, however, arise from underground stems referred to as crowns.
Also, there are many important vegetables in the mustard family. Examples of such vegetables include cabbage, cauliflower, broccoli, kale, brussels sprouts, collards, and kohlrabi. The mustard family also comprises a group of vegetables called the cole crops. This term “cole” perhaps reflects the fact that these vegetables are principally stemmed plants. In cauliflower and broccoli, the flower heads and stalks are eaten. These two plants (cauliflower and broccoli) differ. The white head of the cauliflower is made up of dense clusters of malformed (hypertrophied) flowers. Throughout the growing season, in brussels sprouts, many small heads continually form in the axils of the leaves. The head of cabbage is a large terminal bud.
Another group of angiosperms cultivated for their leaves is Parsley, swiss chard, and spinach. The leek which is a close relative of the onion is cultivated for its leaf bases. There are plants that have underground leafy scales attached to short compressed stems. Such plants are called bulb crops. Instead of storing food in the roots, they store it in the leaves. This causes the leaves to enlarge into bulbs. However, the most common example of such bulb vegetable plants is onions and garlic.
Some angiosperms are cultivated for their roots. Such root crops are cultivated for their fleshy subterranean storage bodies. Examples include bulbs, tuberous roots, corms, rhizomes, and tubers. The potato, for instance, is a tuber belonging to the potato family. Other root crops examples are rutabaga, carrot, turnip, beet, radish, and sweet potatoes.
Another cultivated group of angiosperms is the beans. These include the common bean (Phaseolus vulgaris), navy bean, french or kidney bean, and string bean. They are the edible fleshy pod that contains the bean seeds. These beans provide a good source of protein. Also, Lima beans which likely originated in Central America are now found in Africa, the United States, and the lowland tropics. Moreso, the English or garden pea is cultivated for its edible green pod or seed. The plant is an annual cool-weather plant that is cultivated and found throughout most tropical and temperate regions.
Fruits as vegetables
There are quite a number of fruit vegetables in the family Solanaceae. They include peppers, eggplants, tomatoes, and all herbaceous plants. In the tropics, herbaceous plants are perennial and in temperate zones, they are annual. Pepper plants are grown for their fruits. They include the red or chili pepper as well as the sweet or bell pepper. The bell pepper is usually green when immature but when ripe, it turns red or yellow. Some of the fruits of pepper plants are extremely pungent. This is because of the presence of capsaicin seen in the septa, of the placenta. Also, to a lesser extent, capsaicin is found in the seeds, but not in the fruit wall.
Not only are angiospermous plants grown for their leaves, roots, and fruits. Some are commercially cultivated for their nuts and hard seeds. Such commercially important plants are almonds, pecans, hazelnuts, walnuts, and macadamias. Among the legume family are Peanuts and soybeans. Both plants produce edible seeds that are importantly rich in protein and oil. Also, there are other plants that are rich in oil too. Such economically important plants are the castor bean, coconut, flax, oil palm, olives, sesame, and sunflowers.
Angiosperms vs Gymnosperms
Angiosperms and gymnosperms are both vascular plants that have seeds. However, there are some major differences between these two groups of vascular plants. The major difference between an angiospermous plant and a gymnosperm is their seed development. An angiosperm’s seeds develop in the ovaries of flowers which are surrounded by a protective fruit. The flowers can be unisexual. i.e having male parts and female parts and could also be bisexual by having both male and female parts.
The seeds of gymnosperm, on the other hand, are usually formed in unisexual cones. These unisexual cones are known as strobili. Gymnosperms lack fruits and flowers. However, even though both groups of vascular plants use pollen for fertilization, angiosperms have an exceptional diversity of pollination strategies. Some of such pollination strategies are not seen among the gymnosperms.
Differences between gymnosperms and angiosperms
- Angiosperms are flowering plants whereas, gymnosperms lack flowers.
- Gymnosperms are a smaller ancient group of plants that are considered to be older than angiosperms, even though angiosperms are more successful and dominant on earth. Angiosperms are more dominant than gymnosperms because they have a much larger group of potential pollinators. Hence, they happen to be the most diverse group within the plant kingdom.
- Angiosperm produces their seed which is enclosed in flowers or fruits whereas, gymnosperm produces naked seeds without any protection from flowers or fruits. The naked seeds are not produced by a fruit.
- During reproduction, flowering plants form their seed in the ovary of the flower while gymnosperms form their seeds in cones without any flowers.
- Even though gymnosperms and angiosperms rely on pollination for fertilization, angiosperms have more advantages. The fruits and flowers of the angiospermous plants attract organisms to pollinate and disperse the seed of the plants whereas gymnosperms depend only on natural pollination from storms, water, or wind.
- There are about 300,000 species of extant angiosperms with about 1000 species of extant gymnosperms on earth.
Angiosperms are vascular plants that bear flowers. They are the largest as well as the most diverse group in the kingdom Plantae. These plants represent approximately 80% of all the known extant green plants. The major difference between an angiospermous plant and a gymnosperm is their seed development. An angiosperm’s seeds develop in the ovaries of flowers which are surrounded by a protective fruit. Whereas the seeds of gymnosperm, are usually formed in unisexual cones. These unisexual cones are known as strobili. Gymnosperms generally lack fruits and flowers. Common examples of angiosperms include orchids, grains, flowers, heaths, bulb crops, fruits, timber-yielding plants, vegetables, etc.