Evolution of Seed Habit

Evolution of Seed Habit

One of the most significant events in the history of terrestrial plants was the development of seed habit. It was an essential change in the reproductive system of the vascular plants which occurred approximately 390 million years ago. The first complete seeds appeared roughly 365 million years ago throughout the late Devonian era. Technically a seed may be defined as a fertilized ovule.

An ovule is an integumented indehiscent megasporangium. Integuments are specialized protective coverings around megasporangium which differ in number. All seed-producing plants are called spermatophytes. Various actions involved in the evolution of seed habit are as follows.

1. Evolution of heterospory
2. Retention and germination of megaspore within the megasporangium
3. Development of protective layers around megasporangium
4. Reduction to a single functional megaspore per sporangium
5. Development of an embryo sac within the sporangium
6. 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 approximately pteridophytes are homosporous. During the early phase of evolution, some plant groups began producing two different kinds of spores, the smaller ones called microspores and the bigger ones referred to as megaspores.

The microspores produced inside microsporangia sprout to form male gametophyte or the microgametophyte, whereas the megaspores sprouted to form female gametophyte or megagametophyte.

2.Retention and germination of megaspore within the megasporangium

Throughout the typical 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 the female gametophyte. However, in some plants (e.g., Selaginella) the megaspore is not enabled to get away from megasporangium instantly after its development.

In others, the megaspore is permanently kept within the megasporangium. Here, within the confines of the megasporangium wall, the megaspore sprouts to form an egg including female gametophyte.

3.Development of protective layers around megasporangium

Some branch-like structures of sporophyte surrounding the megasporangium merged around to megasporangium to form a protective envelope or integument. The megasporangium tightly locked by integuments ends up being totally indehiscent. This important change caused 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 structure in the terrestrial environment.

4.Reduction to a single functional megaspore per sporangium

Each megaspore mother cell within a megasporangium utilized to produce four gametophytes. There was a competitor for the area and food amongst the four gametophytes. Quickly the early vascular plants adopted a new method i.e., only one megaspore is chosen 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 consisting of a female gametophyte called an embryo sac.

6.Modification of the distal end of megasporangium for pollen capture

When most of the structural and functional changes leading to the development of seed habit were finished, another essential modification occurred 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 including male gametophyte).

Pollen after being trapped in the distal cavity of the megasporangium produces a pollen tube that carries 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 a maximum degree of protection to a developing embryo under the unfavorable terrestrial environment. The evolution and development of seed habit was an excellent success and a giant leap that eventually made it possible for plants to colonize land permanently.