Evolution of Seed Habit Part 1
One
of the most significant events in the history of land plants was the
development of seed habit. It was an important change in the
reproductive system of the vascular plants which occurred approximately
390 million years ago. First complete seeds appeared approximately 365
million years ago during late Devonian times. Technically a seed may be defined as a fertilized ovule. An ovule is an integument
indehiscent megasporangium. Integuments are specialized protective
coverings around megasporangium which vary in number. All seed producing
plants are called spermatophytes. Various steps involved in the evolution of seed habit are as follows.
- Evolution of heterospory.
- Retention and germination of megaspore within the megasporangium.
- Development of protective layers around megasporangium.
- Reduction to a single functional megaspore per sporangium.
- Development of an embryo sac within the sporangium.
- Modification of distal end of megasporangium for pollen capture.
Primitive vascular land plants produced one kind of spores, a condition called homospory. All groups of land plants up to pteridophytes are homosporous. During the early phase of evolution some plant groups started producing two different types of spores, the smaller ones called microspores and the larger ones known as megaspores.
The
mivropores produced inside microsporangia germinate to form male
gametophyte or the microgametophyte, whereas the megaspores germinated
to form female gametophyte or 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 structure 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 becoming the seed coats. The seed offers maximum degree
of protection to a developing embryo under the unfavorable terrestrial
environment. The development and evolution of seed habit was a great
success and a giant leap which ultimately enabled plants to colonize
land permanently.
2 Comments:
good description
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