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Heterospory and seed habit

This document discusses heterospory and the seed habit in plants. It begins by introducing heterospory as the production of two different types of spores, microspores and megaspores, which is considered a prerequisite for seed formation in plants. It then describes how certain plant species like Selaginella show heterospory, with microsporangia producing many microspores and megasporangia containing just a few megaspores. The highest evolved species of Selaginella, S. apoda, has progressed towards characteristics of seed plants like retaining its single megaspore within the sporangium for fertilization. In summary, the key steps in the evolution of the seed habit involved the development

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HETEROSPORY AND SEED HABIT PRESENTED BY:- PayalS.Chaudhari M.sc Sem 1(botany) Paper : CBO-402 Department of Life Sciences, H.N.G.U PATAN
CONTENT  INTRODUCTION  HETEROSPORY  SEED HABIT
INTRODUCTION Heterospory is considered to be a pre-requisite to seed formation,in addition to heterospory the seed plants show the following characteristic's that have led to the formation of the seed.
HETEROSPORY  The sporangia show greater specialization that those of ferns.  They are differentiated into micro and megasporangia.  The microsporangia posses microsporangia whereas megasporangia contain megaspores in them. Each microsporangium contains a large number of microspores whereas each megasporangium possesses usually only four megaspores.  The condition of the production of two kinds of dissimilar spores,differing in shape and size in the same species is termed heterospory.
 When one kind of spores are present, viz., Equisetum and ferns, the condition is called homospory.  According to modern pteridologists the heterospory is found in seven genera- Selaginella, Isoetes, Marsilea ,Salvinia, Azolla, Regnellidium and Pilularia.
 Here one thing can be noticed that the microspores develop male female gametophytes which may also be called as micro gametophytes and mega gametophytes.  In the case of homospory the sex can be differentiated only at the gametophyte stage, but here the sex may be differentiated even at the saprophytic stage by the presence of small and large sized microspores which form antherozoids and eggs respectively.  The differentiation of microspores and megaspores and their dependence upon the sporophyte has some advantages.
Seed habit  The condition of heterospory in selaginells constitute one of the very important phases in the evolution of the plants.  In certain species of selaginella only one megaspore develops within the mega sporangium.  The highest evolved species, that is, S. apoda has advanced almost upto the rank of a seed plant.
 Here, the megaspore remains within the sporangium; it itself fertilizes and such way for the first time the gametophyte of selaginella show complete dependence of the gametophyte upon sporophyte as in angiosperms.  Biological importance of heterospory: The phenomenon of heterospory is of great Biological advantage due to the fact that a large megaspore which contains female gametophyte derives its food the sporophyte, and is independent of the external conditions as might interfere with the growth of a free living gametophyte.
 In brief, the origin of seed habit is associated with the following important prerequisites : 1. The production of two types of spores (i.e., heterospory). 2. The retention and germination of the fertilization of the egg and embryo formation which is still within germinating megaspore. 3. Development of only one megaspore per megasorengium.
Reference * Botany for degree students - pteridophyta by:Vasishtha B.R Sinha A.K. Singh V.P *www.google.com
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