The below mentioned article provides a study note on Angiosperms – The Flowering Plants.
Angiosperms are those seed plants in which seeds are formed inside fruits and the sporophylls are organised into flowers.
1. Flowering plants or angiosperms are the most recently and highly evolved plants. They appeared on earth about 130 million years ago. First angiosperm fossil recorded from mid cretaceous period is Archaefructus from China and Archaeanthus from Canada.
2. They are the most abundant and conspicuous plants with about 270,000 (220,000 dicots and 50,000 monocots) species.
3. Flowering plants occur in most environments on the earth. Sea is the only exception. Here only two angiosperms are found. They are Zostera and Thalassia. Wolffia is smallest (0.1 cm) while Eucalyptus regnans is largest (114 m height). Angiosperms are most widespread in plains as well as hills.
They have been recorded at an altitude of 6000 m in the Himalayas or permanently frozen Antarctica. Even deserts possess flowering plants. Some of these desert plants are able to grow soon after good shower, flower and shed seeds in a few weeks of wetness available in the area. One species is reported to occur in hot springs at 60°C.
A small orchid lives underground. It has mycorrhizal association which helps it obtain nourishment from decaying organic matter. In moist areas some plants grow perched on other plants. They are dependent upon the larger plant for space only (neither food nor water). Such plants are called epiphytes. Epiphytes are popularly known as space hosts, e.g., Vanda.
4. Sporophylls are aggregated to form flowers. Therefore, angiosperms are also called flowering plants.
5. Both microsporophyIIs and megasporophylls are specialised. A microsporophyll or stamen consists of a filament and an anther. A megasporophyll or carpel is rolled and partly sterilized to produce a stigma, style and ovary containing ovules. Within ovules are present highly reduced female gametophytes known as embryo sacs. Embryo sac formation is preceded by meiosis. So all the cells of embryo sac are haploid.
6. Pollination is through several agencies but most prominent amongst them is by animals, especially insects. The flowers possess showy petals, edible pollen or nectar for this.
7. Pollen grains or microspores reach stigmatic surface found at the tip of carpel or megasporophyll.
8. Female gametophyte or embryo sac develops up to 8-nucleate state prior to fertilisation. There is a three cell egg apparatus (one egg cell or oosphere and two synergids), three antipodal cells and two polar nuclei. The two polar nuclei fuse to form a diploid secondary nucleus.
9. Archegonia are absent. Instead, there is one oosphere surrounded by two specialised synergid cells that attract the pollen tube. The latter brings two naked non-flagellate male gametes.
10. There is double fertilisation. One male gamete fuses with oosphere or egg cell to form zygote. It is called generative fertilization or syngamy. The second male gamete fuses with diploid secondary nucleus to form triploid primary endosperm nucleus (PEN). It represents vegetative fertilization. The latter is also called triple fusion since three nuclei are involved in its formation. Total 5 nuclei take part in double fertilization.
11. Zygote develops into embryo. Central cell containing triploid primary endosperm forms triploid endosperm. Endosperm provides nourishment to developing embryo. Synergids and antipodals degenerate after fertilization.
12. Fertilized ovules ripen into seeds. The seeds are covered by fruits. A fruit is technically a ripened ovary. Fruits not only protect the seeds but also help in their dispersal.
13. Xylem contains vessels.
14. Phloem possesses sieve tubes and companion cells.
15. Secondary growth occurs in stem and root of some angiosperms, placed in group dicots.
Flower is basically a shoot which has been modified in angiosperms for carrying out the process of sexual reproduction. A flower has a limited growth with a long inter-mode at the base (appearing as stalk or pedicel) and condensed nodes present on thalamus or torus. The latter bears four types of structures— sepals, petals, stamens and carpels.
Sepals and petals are sterile floral leaves. Sepals are green and cover the flower in the bud condition. They are protective in nature. Petals are showy and are meant for attracting animal pollinators, especially insects. Stamens are specialized microsporophyll’s. Each stamen has a narrow stalk or filament and a knob-like broader tip celled anther.
Anther develops four microsporangia or pollen sacs. The latter produce microspores or pollen grains. Carpels are specialized megasporophylls. They may be free or fused. Each carpel has a swollen base named ovary.
The interior of ovary contains one or more placentae (singular- placenta) for bearing ovules. The tip of the carpel has sticky tip known as stigma. Stigma is meant for receiving pollen grains. Stigma is connected with ovary through a stalk known as style.
The transfer of pollen grains or microspores to the receptive area of megasporophyll or stigma is called pollination. Cross pollination is common in which pollen grains of one flower reach the stigma of another flower. In gymnosperms, cross pollination is accomplished by wind.
The same is known as anemophily. In angiosperms wind pollination or anemophily takes place in a few cases. In most of the plants, pollination is effected by animal’s insects, worms, birds, bats and even human beings. Insects out-number all other animals as pollinators.
After falling on the stigmas, the pollen grains germinate and develop pollen tubes. The latter descend the styles; enter the ovary and then ovules to perform fertilization. After fertilization the ovules mature into seeds. The ovary containing the seeds ripens into a fruit. The name angiosperm for flowering plants is derived from their characteristics of having covered seeds or seeds enclosed inside the fruits.
The fruit not only protects the seeds during their ripening but also aids in seed dispersal. Some dry fruits have explosive mechanism to throw the seeds. Other dry fruits have devices to stick to the fur of animals or float in air and water. There are other fruits in which the fruit wall is fleshy.
The fleshy ripe fruits attract and tempt animals, especially birds, to eat them. However, seeds of fleshy fruits remain unharmed by their picking or even after passing through their digestive tracts. The animals disperse the seeds during the process of feeding on fruits.
Flowering plants have thus exploited insects, birds and several other animals as agents of pollination, fruits and seed dispersal. It is one major reason for present day dominance of flowering plants.
All angiosperms, however, do not possess seeds. In some cases evolution has resulted in resorting to only vegetative means of propagation because this avoids competition from other plants. The fruits of some such plants are seedless, e.g., Banana.