ALTERNATION OF GENERATIONS 149 



the other from the oosperm. Attention is next drawn 

 to the cytological difference between these two generations, 

 and reference is made to Lloyd WilUams's work on Dictyota 

 pubHshed in 1898, and to Yamanouchi's paper on Poly- 

 siphonia violacea, which appeared in 1906, where it was 

 shown that although the sexual and asexual generations 

 are morphologically identical, cytologically they differ 

 profoundly, the sexual generation, produced from a 

 spore, having x chromosomes in its nuclei and the 

 asexual plant produced from the oosperm having zx 

 chromosomes. Lang considers that these Algae " have 

 an alternation of generations strictly comparable to those 

 in archegoniate plants/' (The crux of the whole position 

 lies in that sentence— are these generations comparable 

 with the sporophyte and gametophyte of the Mosses 

 and Ferns ? ) ''In seeking for an explanation of the 

 great differences between the alternating generations 

 which are characteristic of the Bryophyta and Pterido- 

 phyta it is evident that there are two possible views : 

 (a) that the germ cells are so different that they necessarily 

 give rise to bodies of different structure ; (h) that the 

 two germ cells ahke represent specific cells of the plant, 

 but that the conditions under which they develop are so 

 different that two very unlike bodies result." Lang 

 accepts the second of these alternatives and goes on to 

 explain his theory in the following words : " There is 

 normally a great difference in the conditions under 

 which the spore and the fertiHsed eg^ commence their 

 development in all archegoniate plants. The spore 

 develops free, in direct relation to the soil, water, light, etc. 

 The fertiHsed Qgg, on the other hand, develops in relation 

 to the body of the sexual generation. It thus develops, 

 under profoundly different conditions from the spore, 

 firstly in that it is removed from all the influences acting 

 on the spore, and secondly in that it is exposed to a new 

 set of nutritive and correlative influences proceeding 

 from the maternal body. ... In the case of the spore- 



