Evolution of Seed Habit Part 3

Evolution of Seed Habit Part 3

Class Angiospermae

          The term angiosperms literally means “enclosed seeded” (angio=close sperm=seed). In these plants fertile leaves bearing ovules are folded and joined at the margins to form ovaries. The ovary after fertilization is changed into a fruit, containing seeds.

          Angiosoperms make up 235,000 of the 360,000 known species of plants. They are heterosporous, autotrophic plants. These are highly evolved of all the plants on the earth. The plants produce flowers, fruits and seeds.

(a)

(b)

(c)

Fig. 9.22 some of the remarkable diversity of angiosperms is shown in these photographs. The species shown here are dicots (a) fragrant water lily, (b) wild geranium, (c) Indian pipe (aparsite) an angiosperm that lacks chlorophyll.

Life cycle of an angiospermic plant

            The adult plant is a diploid saprophyte mostly differentiated into roots stem and leaves. At maturity it produces flowers. A flower is a modified shoot which consists of a pedicel, thalamus or torus, and floral leaves (sepals, petals, stamens and carpals). Thalamus and floral leaves, especially the stamens and the carpals, are so modified, that they do not even look like stem and leaves respectively. The sepals and petals are non-essential or non-reproductive parts, and stamens and carpals are the essential or reproductive parts of the flower.

              The sepals and the petals protect the stamens and the carpals. They also attract insects for pollination. When the pollination is over, the sepals usually and the petals always fall off.

          The anther contains microspore mother cells which produce haploid microspores through mitosis. Each microspore germinates to produce male gametophyte. Such microspores containing male gametophytes are called pollen.

               The carpel consists of a basal broader apt, the ovary, the style and the, terminal part of the style. The stigma. The ovary contains ovules. The ovule consists of an integument (covering) and a tissue, the nucleolus present inside.

After pollination, the pollen grain is transferred to the stigma. Here it germinates to form a pollen tube. The nucleus of the microspore divides by mitotic divisions to form two male gametes and the tube nucleus. At this stage of development, the pollen grain is called male gametophyte. In the meantime certain changes occur in the ovule leading to the formation of female spore (megaspore). The megaspore develops into female gametophyte. This consists of seven cells only. One of these cells is the egg of oosphere.

The pollen tube grows through the style, enters the ovule and then reaches the female gametophyte. Here it discharges the male gametes. The egg and one of the two male gametes fuse to form the oospore. The second male gamete fuses with the secondary nucleus to form endosperm nucleus (double fertilization). The oospore develops into an embryo and endosperm nucleus develops into a multicellualr nutritive tissue, the endosperm.

Seed formation

          Meanwhile, the integuments of the ovule form testa and tegmen and ovary wall develops into the fruit. Seeds usually undergo a period of rest and then under suitable conditions, germinate and produce a seedling which gradually changes into a sporophyte (fig. 9.23).

Thus an alternation of dominant sporophyte generation (2n) occurs with inconspicuous gametophyte generation (n).

Fig. 9.23 life cycle of on angiospermic plants.