Page 22 - 11-Bio-9 Kingdom plantae

9. Kingdom Plantae eLearn.Punjab

megagametophyte.

2. Retention and germination of megaspore within the megasporangium

During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores
are used to be shed and dispersed soon after their formation in order to germinate into female
gametophyte. However in some plants (e.g. Selaginella) the megaspore is not allowed to escape
from megasporangium immediately after its formation. In others the megaspore is permanently
retained within the megasporangium. Here, within the confines of the megasporangium wall the
megaspore germinates to form egg containing female gametophyte.

3. Development of protective layers around megasporangium

Some branch like structures of sporophyte surrounding the megasporangium fused around to
megasporangium to form protective envelope or integument. The megasporangium tightly locked
by integuments becomes totally indehiscent. This important change led to the evolution and
formation of the ovule, which is nothing but an integumented indehiscent megasporangium. In
this way more protection is accorded to the egg-containing apparatus in terrestrial environment.

4. Reduction to a single functional megaspore per sporangium

Each megaspore mother cell within a megasporangium used to produce four gametophytes. There
was a competition for space and food among the four gametophytes. Soon the early vascular
plants adopted a new strategy i.e., only one megaspore is selected for further development into a
healthy female gametophyte while the remaining three are aborted.

5. Development of an embryo sac within the sporangium

The single healthy megaspore retained within the megasporangium germinates to form an egg
containing female gametophyte called an embryo sac.

6. Modification of distal end of megasporangium for pollen capture

When most of the structural and functional changes leading to the development of seed habit were
completed, another important modification took place in the megasporangium which was now
integumented, indehiscent and permanently attached to - the sporophyte. The distal end of the
megasporangium became modified for capturing pollen (microspore containing male gametophyte).

Pollen after being trapped in the distal cavity of the megasporangium produces pollen tube which
carry male gametes deep into the embryo sac to fertilize the egg, forming a zygote, that forms an
embryo. The megasporangium (ovule) after fertilization is transformed into a seed, the integuments

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