XV NATURE OF THE ALTERNATION OF GENERATIONS 567 



In the simplest sporophyte of the Liverworts as ilkistrated 

 by Riccia, there is first the separation of the superficial layer of 

 sterile cells, about the central mass of sporogenous tissue, and 

 each cell of the latter produces four thick-walled resting spores, 

 corresponding physiologically to the single resting spore of the 

 Alga. The retention of the zygote within the archegonium and 

 the parasitic habit of the embryo developed from it enables the 

 sporophyte to reach a much larger size than is possible where 

 the germination is entirely at the expense of the food-materials 

 stored up within the spore, as is necessarily the case where the 

 zygote becomes free before germination, as it does in all the 

 Chlorophyceae. When to this is added the division of each spo- 

 rogenous cell into four spores, it is clear that the output of 

 spores resulting from a single fertilisation is very much 

 increased, a great advantage for a terrestrial plant in which the 

 conditions for fertilisation may not occur very often. 



The formation of the spores in tetrads is common to all 

 Archegoniates, and it is preliminary to this division that there 

 occurs the reduction in the number of the chromosomes which 

 has been observed in a number of cases. While this reduction 

 is not always strictly definite, it is found that the spore has 

 approximately one-half the number of chromosomes which are 

 found in the vegetative cells of the sporophyte, and this reduced 

 number, of course, is transferred to the tissues of the gameto- 

 phyte which arises from the germination of the spore. When 

 the gametes fuse, the zygote-nucleus receives the combined 

 chromosomes of the gametes, and the sporophytic cells de- 

 scended from it contain the double number of chromosomes. 



We must assume that in its primitive form the sporophyte 

 of the first Archegoniates was composed exclusively of spo- 

 rogenous tissue, as it is in Colcochcctc. Riccia shows the first 

 indication of the sterilisation of the outer layer of sporogenous 

 tissue. Professor Bower (16) has called attention to the great 

 importance of the principle of sterilisation of potentially spo- 

 rogenous tissue in the evolution of the sporophytic structures 

 among the Archegoniates 



The next step in the evolution of the sporophyte, as it is 

 seen in the Liverworts, is one of great importance in the further 

 evolution of the sporophyte. This is the sterilisation of the 

 whole of the basal part of the sporophyte, which assumes the 

 important role of a special organ of absorption, or haustorium. 



