6o 



MORPHOLOGY 



nuclei that have descended from the fusion nucleus. The irregular 

 fusion chamber puts out lobes, into each one of which a nucleus passes 

 and divides. At the tip of each lobe a spore is developed by a sort of 

 budding, and into it one of the two lobe nuclei passes. In this way sixty 

 or more carpospores are formed in a single cystocarp. The usual en- 

 velope of sterile cells, in this case an urn-shaped envelope, is developed 

 about the whole carpospore-bearing structure from the adjacent cells 

 (fig. 150). 



Tetrasporic plant. Upon germination the carpospores give rise to 

 plants that produce tetraspores, but no sex organs. The sporangia 



arise laterally from the 

 axial siphon, each spo- 

 rangium standing on a 

 stalk cell and finally 

 pushing through the 

 covering of cortical cells 

 (fig. 151). Upon germi- 

 nation the tetraspores 

 give rise to plants that 

 bear sex organs (anthe- 

 ridia and procarps). 



Alternation of genera- 

 tions. This remark- 

 able life history intro- 

 duces us to the alterna- 

 tion of generations, a 



r ,.r u:,, tnr ^ t 



152 153 



FIGS. 152, 153. Polysiphonia: 152, diagram show- 

 ing formation of antheridia (a); 153, diagram showing 

 structure of young procarp (pc, pericentral ; ct, carpo- IS invariable in the great 

 gonium and trichogyne ; a, cells producing the auxiliary plant groups above 

 cells, one of which crowds in between the carpogonium ,1 n u TJ . 



and the pericentral).- After YAMANOUCHI. thallophytes. It IS an 



alternation of sexual 



and sexless individuals, each producing spores that give rise to the 

 other. The cytological test of alternation, referred to under Coleo- 

 chaete, has been applied to Polysiphonia, and the fact of a real alterna- 

 tion has thus been established. It will be remembered that in such an 

 alternation the number of chromosomes characteristic of the nuclei is 

 doubled by the act of fertilization; therefore, the nuclei of the sexless 

 individuals (sporophytes), which are products of fertilization, contain 

 the double number (2*) of chromosomes; while the nuclei of the sexual 



