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 
FIGS. 152, 
152 153 
. Polysiphonia: 152, diagram show- 
51 53- ~ " tvpe of life historv that 
ing formation of antheridia (a); 153, diagram showing m j " 
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 , T , . 
andthepericentral).-AfterYAMANOUCHi. 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 
