xv NATURE OF THE ALTERNATION OF GENERATIONS 575 



Farmer's recent remarkable studies on apogamy (Farmer 

 (10)), show that nuclear fusions occur, indicating that a stim- 

 ulus, equivalent to fertilisation, is necessary for the develop- 

 ment of apogamous structures. 



It would seem then, that the adaptation to strictly terrestrial 

 conditions, and the consequent necessity for providing an ade- 

 quate water supply, is the real clue to the causes for the develop- 

 ment of the leafy sporophyte. All Bryophytes retain to some 

 extent the character of aquatic plants, most of them being able 

 to absorb water at all points, and relying only to a limited extent 

 upon the rhizoids. Moreover, the latter are entirely inadequate 

 to supply a plant-body of large size; which could not, of course, 

 absorb sufficient water for its growth from the atmosphere. 

 Nature 'has apparently made numerous attempts to adapt the 

 essentially aquatic gametophyte to an aerial existence, with only 

 partial success. 



The sporophyte, at first purely a spore-producing structure, 

 was from its inception essentially an aerial organism. Its 

 water supply from a very early period was furnished through 

 the agency of the massive foot, which drew upon the gameto- 

 phyte for its supply, and formed a much more efficient haus- 

 torium than the rhizoids of the gametophyte. Later was 

 developed a true root, probably a modification of the foot, but 

 unlike the latter, connecting the sporophyte with the earth. 



With the appearance of the first true root, the emancipation 

 of the sporophyte is complete, and as the root system develops 

 to keep pace with the aerial parts of the sporophyte, a true ter- 

 restrial type of plant is encountered for the first time. The 

 appearance of the first genuine green land plants may be con- 

 sidered the most momentous epoch in the whole history of the 

 Plant Kingdom. 



