A review of the kingdom plantea indicates that the seed-plants (spermatophytes predominate over non-seed vascular plants.
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.
1. Evolution of heterospory
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.