124 THE EVOLUTION OF PLANTS 



admitted that, so far as fossil evidence shows, 

 heterosporous plants go back just as far as the 

 homosporous. That, however, may be because 

 the record has lost all its earlier chapters. 



We may regard the differentiation of male 

 and female spores (microspores and megaspores) 

 as the first step towards the evolution of repro- 

 duction by seeds. In the Palaeozoic, all the Ly- 

 copods (Club-mosses) of which the reproduction 

 is sufficiently known, have proved to be hetero- 

 sporous; so also were a portion of the Calami tes 

 (Palaeozoic Tree-horsetails); curiously enough 

 no Ferns with two kinds of spores have yet been 

 discovered in the Primary rocks. 



The heterosporous arrangement, as we find 

 it in Cryptogams, has difficulties of its own, 

 which to some extent balance its advantages. 

 In order that fertilisation may be accomplished 

 it is necessary for the two kinds of spores to 

 germinate together, as well as in the presence 

 of an adequate water supply. This necessary 

 association of the large and small spores, which 

 often differ in mass in the proportion of 100,000: 1, 

 if left to chance, is a very precarious matter, for 

 bodies of such different weights, if carried by 

 the wind, will be unlikely to come to rest to- 

 gether. Hence a vast number of microspores are 

 necessary a somewhat expensive arrangement. 



One means of ensuring greater precision in 



