Angiosperms

Angiosperms represent the most advanced group of vascular plants. They are commonly called 'Flowering plants'. They exceed all other major groups of living plants in number and diversity.

Angiosperms grow in almost every kind of habitat. They occur in very high altitudes, even in Antarctica, in deserts, in shallow waters and even on other plants as parasites.

The adult plant body of an Angiosperm is a sporophyte with a well defined root, stem and leaves. The root may be a taproot or fibrous root. The stem may soft and green (herbaceous) or hard and woody. The leaves may be simple or compound.

The most important feature in angiosperms is that they exhibit reproductive structures called flowers. The flowers contain certain structures arranged in four whorls. The outer whorls contain accessory structures such as sepals and petals and the two inner whorls containing essential structures such as stamens and carpels.

The stamens represent microsporophylls. Each stamen has an anther and a filament. The anther produces pollen grains containing the highly reduced male gametophyte. The carpels represent megasporophyll. They enclose ovules containing the egg cell which with the associated cells represent the female gametophyte.

A characteristic feature of angiosperms is the occurrence of a phenomenon called double fertilisation one male gamete fuses with the egg cell to form a zygote while another male gamete fuses with a dipolid secondary nucleus, to form a triploid endosperm (triple fusion). Following pollination and fertilization, the ovule transforms into the seed while the ovary that encloses the ovule, transforms into the fruit. Thus, seeds are enclosed in fruits.

fig. 8.34 - Life Cycle of an Angiosperm

Life Styles in Angiosperms

Angiosperms exhibit a great diversity in form, size, structure and life span of the plant body. Based on the duration of life span, angiosperms are classified into annuals, biennials and perennials.

Annuals

• are plants which complete their life cycle in one season.

E.g., Sunflower, Mustard, Pea, Bean, Rice, Maize.

Biennials

• are plants that complete their life cycle in two years. In the second year of their life span they produce flowers, fruits and seeds.

E.g., Carrot, Radish, Beetroot, Turnip.

Perennials

• are plants that live for more than two years. Every year they produce flowers, fruits and seeds.

E.g., Mango, Rose, Dahlia, Bamboo.

Annuals

Biannuals

Perennials

fig. 8.35 -Examples of Angiosperms

Based on the habit, nature and height of the stem and duration of life cycle, angiosperms can be distinguished into herbs, shrubs and trees.

Herbs

• are small plants with a soft and green stem. They reach a height of not more than five feet and are mostly annuals or biennials.

Shrubs

• are woody perennial plants that may reach a height of five to ten feet. Stem is often profusely branched giving a bushy appearance.

Trees

• are woody perennials which reach considerable heights. The stem is hard, woody and is called trunk. It may be unbranched as in palm trees or extensively branched as in most others.

Some trees like mulberry and oak, shed all their leaves at the peak summer or winter season. Such trees are described as deciduous. Many other trees like mango and guava do not shed their leaves. Such trees are described as evergreen.

Major groups of Angiosperms

Irrespective of any system of classification that is being used, angiosperms are divided into two natural groups called monocotyledons (or monocots) and dicotyledons (or dicots). This distinction is based on the number of embryonic leaves (or cotyledons) in the embryo. The following table summarises the differences between monocots and dicots.