INTRODUCTION 15 



The first concerns the nutrition of the prothallus and the 

 subsequent embryonic sporophyte. The retention of the 

 gametophyte within a resistant spore wall severely Hmits its 

 powers of photosynthesis and may even prevent it alto- 

 gether. Hence, it is necessary for such a prothallus to be 

 provided with abundant food reserves; the larger the spore, 

 the more that can be stored within it. This may well account 

 for the lage size of the spores which are destined to contain 

 an embryo sporophyte, but it does not explain why the pro- 

 thalli should be unisexual (dioecious). This is most probably 

 concerned with out-breeding. It is widely accepted that any 

 plant which habitually undergoes inbreeding is less likely to 

 produce new varieties than one which has developed some 

 device favouring out-breeding, and that such a plant is at a 

 disadvantage in a changing environment. It will tend to lag 

 behind in evolution. Now, monoecious gametophytes (bear- 

 ing both archegonia and antheridia) are much more hkely to 

 be self-fertihzed than cross-fertilized, unless they are actually 

 submerged in water. Yet, dioecious prothalli in a terrestrial 

 environment would be at an even greater disadvantage, for 

 they might never achieve fertilization at all, so long as the 

 antherozoid has to bear the whole responsibility of finding 

 the archegonium. This is where heterospory may operate to 

 the advantage of plants with dioecious prothalli. Those 

 spores which are destined to produce male prothaUi need 

 not carry large food reserves and can, therefore, afford to be 

 reduced in size to the barest minimum. From the same initial 

 resources, vast numbers of microspores can be produced and 

 this will allow some of the responsibility for reaching an 

 archegonium to be transferred to them. Blown by the wind, 

 they can travel great distances and some, at least, will fall on 

 a female prothallus in close proximity to an archegonium. 

 Thus, when the male prothallus develops, the antherozoids 

 Uberated from the antheridia have only a short distance to 

 swim and, in order to do so, need only a thin film of 

 moisture. Under ordinary circumstances, the chances may 



