240 PLANT LIFE 



somes. It invariably happens, so far as at 

 present is known, that when the eggs are 

 fertile at all they produce new ferns 1 directly, 

 that is, without fertilisation. Moreover, even 

 the tissue cells of such a prothallus may 

 change their mode of growth, and develop 

 into fern plants without the definite produc- 

 tion of sexual organs at all. 



Such a departure from the normal course 

 of life history strongly emphasises the relation 

 of meiosis to fertilisation, but it does more 

 than this. It indicates that the striking 

 difference between the fern plant and the 

 prothallus is not itself essentially bound up 

 with those nuclear changes which are inti- 

 mately associated with the sexual phases. 

 It points rather to the conclusion that in these 

 plants the life history, with its two different 

 stages, may have developed in coincidence, 

 though not in causal connection with the 

 separation of the sexual process into two 

 stages. It would clearly be futile, in the face 

 of the evidence, to attempt to maintain the 

 existence of a causal relation between the 

 nuclear changes and the characteristic differ- 

 ences between the two stages of the life 

 history of the fern. In this way we may 

 understand the continuance of the alternate 

 appearance of fern and prothallus, even when 

 the cellular rhythm no longer obtains. 



Considerations of space preclude the follow- 

 ing up of this matter in any detail; it may, 

 however, be said quite generally that wherever 



