In this article we will discuss about:- 1. Origin of Angiosperm 2. Size of Angiosperm 3. Habit 4. Life Span 5. Habitat 6. Nutrition.
Origin of Angiosperm:
Flowering plants or angiosperms are the most recent, most advanced, most evolved, most conspicuous and abundant of all the plants. They appeared about 130 million years back but comprise about 3, 00,000 species or 50% of all plants. They show great diversity in their form, size, life span, habit, habitats, nutrition, etc.
Size of Angiosperm:
The smallest angiosperm is a rootless aquatic Wolffia. It has a diameter of 0.1 mm. Aquatic Lemna has a diameter of 0.1cm. Arceuthobium is another small angiosperm plant which is parasitic over Pinus and other trees.
The tallest plant is Eucalyptus regnans. It is over 100 m tall. Certain specimens of this plant have reached a height of 130.5 m. The largest sized plant is Banyan tree (Ficus benghalensis). It cans spread over an area of 2-5 acres.
Habit of Angiosperm:
It is determined by height, duration and nature of stem. In habit the plants are of three main categories- herbs, shrubs and trees. Other types are trailers, creepers, twiners, climbers, lianas and epiphytes.
1. Herbs:
They are small plants with soft and pliable stems which normally grow to a height of less than 2m. Herbs may be annual (e.g., Wheat, Buttercup), biennial (e.g. Henbane) or perennial (e.g., Canna). The perennial herbs have either water conserving mechanism or possess underground stem which produces new shoots every year.
2. Shrubs:
Plants are of medium height with perennial woody stems. A trunk is absent. Many stem branches of equal height may arise from near the base. They are woody below and herbaceous near the apices. Shrubs are also called bushes, e.g., Capparis, Jasmine, and Rose.
3. Trees (Fig. 5.1):
Plants are of great height with a thick woody main stem called trunk. The trunk may remain un-branched when it is called caudex or columnar, e.g., Palm.
In ex-current form the trunk produces narrow lateral branches which do not compete with the former. The appearance is cone-like, e.g., Pinus, Casuarina, and Eucalyptus. In deliquescent type the trunk disappears after some distance so that the crown appears dome shaped, e.g., Dalbergia, Banyan.
4. Trailers spread over the ground without rooting of prostrate stems, e.g., Tribulus, Euphorbia.
5. Creepers have prostrate stems which root at intervals, e.g., grass.
6. Twiners are weak stemmed plants where stems twine around the support, e.g., Ipomoea.
7. Climbers rise up their support by means of special clinging or coiling structures e.g., Grape Vine.
8. Lianas are woody twiners or climbers, e.g., Hiptage, Phanera.
9. Epiphytes are plants that live on other plants for space, e.g., Vanda.
Life Span of Angiosperm:
On the basis of life span, plants are of three types— annual, biennial and perennial.
(1) Annuals:
The plants complete their life cycle in a single season of a few weeks to a few months, e.g., Wheat, Maize, Pea, Gram, Euphorbia prostrata.
(2) Biennials:
The plants live for two seasons (spread over more than one year). In the first season they grow vegetatively and store food. Next season they produce flowering shoots, e.g., Henbane. Radish, Carrot and Turnip are biennial in colder areas. They become annual in warmer places.
(3) Perennials:
The plants live for a few years to several hundreds of years. These plants usually bear fruits every year when they are called polycarpic. In some bamboos and Agave, flowers and fruits are formed only once after a vegetative growth of several years (Some 40 years in Century Plant or Agave Americana).
They are called monocarpic. Some of the perennial plants have been turned into annuals under cultivation (e.g., Cotton) while others do so under change of climate (e.g., Castor). Bodhi tree (Ficus religiosa) at Gaya ji is over 2500 years old. Specimens of Larrea tridentata are 11, 300 years old while that of shrub Lomantia tusmanica is 43,000 years old.
Habitat of Angiosperm:
It is a natural home or abode of the organisms. Angiosperms are mostly terrestrial. A few are aquatic. The aquatic forms are called hydrophytes. In sea only two angiosperms are recorded. They are Zostera and Thalassia.
More angiosperms occur in fresh water, e.g. Wolffia, Lemna, Pistia, Hydrilla, Nelumbo, Trapa. Plants of terrestrial habitats are of several types. Mesophytes are plants of moist habitats like tropical rain forests, many crop plants and ornamentals, e.g., Sunflower, Artocarpus, and Mulberry. Xerophytes are plants of dry habitats, e.g., Alhagi, Capparis, Acacia.
Some of them store mucilage and water. They are called succulents, e.g., Euphorbia, Opuntia. Psammophytes are terrestrial plants which occur in sandy habitats. Lithophytes are plants growing over rocky substrata. Halophytes are plants of saline habitats. They may be terrestrial (e.g., Salsola, Sueda) or found in marshy habitats along sea shore. The latter are called mangrove plants, e.g., Rhizophora.
Nutrition of Angiosperm:
Angiosperms are mostly autotrophic plants or autophytes as they are green and capable of manufacturing their own organic food from inorganic raw materials with the help of chlorophyll and sunlight.
A few angiosperms are heterotrophic. They obtain a part or whole of their nourishment from outside. Heterotrophs are of three types- saprophytes, parasites and insectivorous. Saprophytes obtain their nourishment from dead decaying organic matter.
They are also called humus plants e.g., Monotropa, Neottia. Parasites obtain their food requirements from other living organisms. Holoparasites or total parasites get water, minerals and organic food from their host. Cuscuta is a total stem parasite. Rafflesia is a total root parasite. Hemi-parasite or partial parasites get a part of their nourishment from host, e.g. water and mineral salts.
Viscum (Mistletoe) is partial stem parasite and Santlum (the largest parasite) is partial root parasite. Insectivorous plants are green autotrophic plants which supplement their nitrogen supply by catching and digesting small animals. Insectivorous plants commonly grow in nitrogen deficient habitats e.g. Drosera, Nepenthes, Utricularia.