The Evolution Of Seed Habit

EVOLUTION OF SEED HABIT:

Dominancy of Seed Plants Over the Non-seed Plants:

A review of the kingdom Plantae indicates that the seed-pants (spermatophytes) predominate over non-seed vascular plants.

Development of Seed Habits:

One of the most significant events in the history of land plants was the development of seed habit.

Change in Reproductive Structure:

It was an important change in the reproductive system of the vascular plants which occurred approximately 390 million years ago.

First Seed Appear in Late Devonian:

First complete seeds appeared approximately 365 million years ago during late Devonian times.

Seed is Fertilized Ovule:

Technically a seed may be defined as a fertilized ovule.

Ovule is Immature Seed:

Integument Indeshiscent:

An ovule is an integumented indehiscent (not discharge or ripe) megasporangium. 

Integuments are specialized protective coverings around megasporangium which vary in number.

Spermatophytes:

All seed-producing plants are called spermatophytes.

Various steps involved in the evolution of seed habit are as follows.

Steps Involved in Evolution of Seed Habit:

Summary:

(i) Evolution of Heterospory.

(ii) Retention and germination of megaspore within the megasporangium.

(iii) Development of protective layers around megasporangium.

(iv) Reduction to a single functional megaspore per sporangium.

(v) Development of an embryo sac within the sporangium.

(vi) Modification or distal end of megasporangium for pollen capture.

In Detail:

(i) 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.

         (a) Smaller ones called microspores and
         (b) Larger ones are known as megaspores.

• The microspores produced inside microsporangia germinated to form male gametophyte or microgametophyte.

• The megaspores germinated to form female gametophyte or megagametophyte.

(ii) Retention and Germination of Megaspore within the Megasporangium:

• During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores 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 was not allowed to escape from megasporangium immediately after its formation.

• In other, the megaspore(s) was/were permanently retained within the megasporangium.

• Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophyte.

(iii) 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 was accorded to the egg containing apparatus in terrestrial environment. 

(iv) Reduction to a Single Functional Megaspore per Sporangium.

• Each megaspore mother cell within a megasporangium used to produce four functional megaspores.

• These megaspores germinated to produce four variable female 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 was selected for further development into a healthy female gametophyte while the remaining three were aborted.

(v) Development of an Embryo Sac within the Sporangium.

• The single healthy megaspore retained with the megasporangium germinated to form an egg containing female gametophyte called an embryo sac.

(vi) Modification or 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 produced pollen tube which carried male gametes deep into the embryo sac to fertilize the egg, forming a zygote, which forms an embryo.

• The megasporangium (ovule) after fertilization is transformed into a seed, the integuments becoming the second coats (testa).

• 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 giant leap which ultimately enable plants to colonize land permanently.