Evolution of Seed Habit Part 3
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.
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