THALLOPHYTES 



31 



Fig. 83. — Bulbochaete: 

 showing a dwarf male fila- 

 ment, with its terminal 



form. It belongs to the fresh waters and is found 



attached to the leaves and stems of various 



aquatics, as water lilies, etc. The body is a flat 



thallus, being either a complete disk composed 



of radiating rows of cells (fig. 84) or a cushion 



with free branches. The zoospores are sohtary, 



biciliate, and may be produced by any vegetative 



cell (fig. 89). In the discoid species the antheridia 



are formed by the division of a vegetative cell into 



four cells, each one of which produces a biciliate 



sperm (figs. 85, 86). In the branched forms, the 



antheridia appear as special club-shaped cells at 



the ends of branches. In the discoid forms the 



oogonia are near the ends of the radiating rows 



of vegetative cells, differing from them chiefly in 



size (figs. 84, 87). In the branched forms, the 



oogonia are free at the ends of branches, each 



developing a long tubular prolongation through antheridium, attached to 



which the sperm enters. ^" oogonium. — After 



„ ... . , . 1 . , ,1 , Pringsheim. 



Fertihzation results not only m a thick-walled 



oospore, but in the branched forms contiguous cells of the thallus send 

 out branches which invest it in a cellular case, making a sort of spore 

 case (sporocarp), which is the resting stage of the plant. In germina- 

 tion the oospore gives rise to a several-celled body, each cell of which 

 produces a zoospore that escapes and develops a new thallus (fig. 88). 



Alternation. — Two noteworthy facts 

 in this life history are the formation of 

 a case of sterile cells about the oospore 

 as a result, of fertilization, and the 

 multicellular body produced by the 

 oospore. This last fact has been taken 

 to represent the alternation of genera- 

 tions which is established as a constant 

 feature of the higher plants. This 

 phenomenon consists of the alternation 

 of a sexual and a sexless generation in 

 a life cycle, each generation giving rise 



Fig. 84. — Coleochaek scutaia: dis- ^ ^, ^, t ^ 7 7 j -i. 



coid form; the two larger and shaded tO the Other. In Colsochaete it waS 



cells are developing as oogonia. supposed that the sexual generation 



