ALTERNATION OF GENERATIONS 317 



prothallus of a fern is extremely delicate, only one cell thick 

 over most of its area, and quite incapable of withstanding 

 desiccation. It can live only in moist stations. In the 

 Lycopodiaceae and Ophioglossaceae the gametophyte is 

 tuberous, and is in many cases saprophytic and subterranean 

 with a mycorhizal fungus. For good development it 

 requires moist soils rich in humus. An independent 

 gametophyte therefore at once limits the plant to stations 

 where the moisture relations are very favourable. The 

 evolution of a type of gametophyte with drought-resisting 

 capacity could be imagined ; but it has not occurred. 

 Moreover, water is necessary for another reason. The sperm 

 can swim to the ovum only in free water. Fertilisation 

 cannot take place unless surface water is available. Only 

 by living in a constantly moist situation can the plant be 

 assured of a supply of water at the critical point in its life 

 cycle. Now, as the sporophyte generation is a sedentary 

 organism growing where the zygote, embedded in the 

 tissues of the gametophyte, has started to develop, it follows 

 that the sporophyte, too, however marked drought resisting 

 capacity it might possess, would, from its individual point 

 of origin, be limited to moist stations. In fact the abundant 

 remains of the once dominant flora of ferns and their allies 

 which persist to the present day are characteristic of damp 

 situations, the shade of woods, marshes, and slow streams. 

 A few exceptions like the bracken are known to multiply 

 for the most part vegetatively by branching rhizomes. 



The alternative to the evolution of a drought-resisting 

 gametophyte is the reduction of the sexual stage. In the 

 plants where the process is complete we find the utmost 

 possible emancipation from the necessity of a steady or 

 abundant water supply. Reduction is the primary condi- 

 tion, and it has been accompanied by the evolution of the 

 specialised absorbing, conducting, and economising systems 

 of the sporophyte which we have already studied. 



