MICHIGAN ACADEMY OF SCIENCE. 



69 



Certain cells of these diploid, tetrasporic plants enlarge and their 

 nuclei undergo meiotic division producing the lour nuclei of the 

 four tetraspores, each of whidi in turn may produce a new sexual 

 plant. 



5 6 



Figure .5. Sexual cycle of such Florideae as possess a distinct tetrasporic generation. C= 



Point at which carpospores are formed. 

 Figure 6. Sexual cycle of Xenuilion. 



In the life cycle of these plants we find a large number of nuclear 

 and cell generations occuring between the nuclear union (NU) 

 and reduction division (RD) and again between the latter and 

 the formation and union of the gametes, this being illustrated in 

 the figure. 



In some of the Florideae, e. g., Nemalion, there is no tetrasporic 

 generation and the carpospores possess haploid nuclei (Fig. 6). 

 Wolf has shown for these that the zygote nuclei and those first 

 entering into the threads which produce the carpospores are dip- 

 loid, but that the chromosome number becomes reduced somewhere 

 along the course of this tliread so that the last division which 

 l)roduces the carpospores shows the nucleus to be haploid. Thus 

 in the ratlier closely related ]»lants represented on the one hand 

 by Nemalion and on the other, for example by Polysiphonia, the 

 reduction division precedes the carpospore production or follows 

 long after, respectively. 



In the Bryophyta the alternation of generations becomes fairly 

 well marked. In these plants the sexual cycle (Fig. 7) is much 

 like that in Nemalion except tliat the number of cell generations 

 is vastly greater between the zygote and the reduction divisions 

 tliat take place, just before spore production, in the spore mother 



