6o 



MORPHOLOGY 



nuclei that have descended from the fusion nucleus. The irregular 

 Xusion 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 s ixty 

 or more carpospores are formed in a single cystocarT) . 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- iS°)- 



Tetrasporic plant. — Upon germination the carpospores give rise to 



plants that prod uce tetraspores, but no sex orgap s. 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 alterqji- 

 152 153 t ion of prpnprf^tinnf^, a 



Figs. 152, 153. -Polysipkonia: 152, diagram show- ^j jj^^ history that 



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



structure of proearp {pc, pericentral ; rf, carpogonium and IS mvanable m the great 



trichogyne; a, some of the auxiliary cells, one of which plant groups above 



crowds in between the carpogonium and the peri- ,, ,, , -r . 



central). - After Yamanoochi. thallophytes. It IS an 



alternation of sexual 

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

 other. The_c ^logical test of alternation, referred to under Coleo - 

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

 t ion has thus been establi shed^ 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 



