THE SPERMATOPHYTA : GENERAL INTRODUCTION 



639 



The Evolution of the Seed 



In the previous groups of plants which we have considered, the principal 

 organ of dispersal is the spore, the germination of which produces a 

 gametophyte. 



In the lower types this gametophyte is often an independent plant, which 

 in many cases, especially among the Algae, is structurally similar to the 

 sporophyte. An evolutionary process of reduction is, however, traceable in 

 the history of the gametophyte, a reduction both in size and in independence, 

 so that in Selaginella we see a stage at which the gametophytes, which have 

 become unisexual, remain enclosed within the walls of the spores which 

 produce them. The female gametophyte, inside 

 the megaspore, is, however, much bigger than 

 the male and is provided with a store of reserve 

 food material. The growth of the embryo from 

 such a gametophyte is largely external to the 

 prothallus, and its establishment as an inde- 

 pendent plant takes place directly and without 

 any intervening period of rest. 



The essential change which led from this 

 condition to that of the Spermatophyta was 

 the retention of the megaspore within the 

 megasporangium and its fertilization in situ. 

 One or two species of Selaginella, notably 

 S. apoda, show this actual change (Fig. 648). 

 Only one megaspore is formed in each mega- 

 sporangium and it germinates without being 

 shed. The young embryos develop in the 

 shelter of the sporangial wall and of the sporo- 

 phylls, and are only dropped when they are 

 already provided with a root and a primary 

 shoot. 



While this illustrates clearly an intermediate stage in the evolution of the 

 seed it does not make S. apoda a true seed plant, because a further step is 

 required by which the female prothallus, the megaspore and the megasporan- 

 gium eventually become integrated into a new compound structure, the 

 seed, which functions as a biological unit and is detached from the plant 

 and dispersed as a unitary structure. 



The equivalence of the female reproductive structures between the 

 Spermatophyta and the Pteridophyta may be summed up in the following 

 terms. 



In the Pteridophyta we have four separate entities ; the female prothallus, 

 the megaspore, the megasporangium and the megasporophyll. 



In the Spermatophyta the female prothallus is permanently retained in 

 the megaspore, the two together constituting the embryo sac. The mega- 

 spore is likewise permanently retained in the megasporangium, which becomes 



Fig. 648. — Selaginella apoda, 

 showing sporelings from mega- 

 spores which are still in situ on 

 the strobilus. {After Lyon.) 



