HOMOSPORY AND HETEROSPORY 321 



ditions of nutrition are abnormal (cf. p. 313). This tendency 

 leads to the development of definite unisexual prothalli in the 

 Horsetails, which, however, like the Ferns, have only one kind 

 of spore, i.e. are homosporous. Lycopodium resembles the Ferns 

 in these respects, but in Selaginella, not only are the prothalli 

 unisexual, but they are produced from two kinds of spores, 

 i.e. this genus is heterosporous. 



Although heterospory involves the risk of the two sexes of 

 prothalli not germinating in sufficiently close proximity to one 

 another, certain advantages obviously accrue from it. The 

 most conspicuous of these is similar to that which may have led 

 to the evolution of oogamy (cf. p. 223), viz. the provision by the 

 mother-plant of an abundant store of food for the development 

 of the new sporophyte. These reserves are laid down in the 

 megaspore before it is shed, and, as a consequence, the resulting 

 prothallus can dispense with rhizoids and an assimilatory me- 

 chanism. Moreover, the embryo acquires additional protection 

 during the early stages of its development from the coats of the 

 megaspore, within which the greater part of the prothallus 

 remains enclosed (cf. Fig. 182, b.). A further step would 

 obviously be the retention of the megaspore within the spo- 

 rangium until after fertilisation and during the development of 

 the embryo, and this we shall find realised in the Phanerogams, 

 the last great class of the Vegetable Kingdom. The microspores, 

 requiring no appreciable amount of food-reserves, and being 

 consequently of small dimensions, can be produced in large 

 numbers. This affords an increased power of dispersal whereby 

 the association of the two prothalli is rendered more probable. 



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