THALLOPHYTES 



3 1 



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 solitary, 

 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 FlG - 8 3-~ Bulbochaete: 



. showing a dwarf male fila- 



oogoma are free at the ends of branches, each ment> with its term i na i 



developing a long tubular prolongation through antheridium, attached to 



which the sperm enters. an oogonium. After 



T-. i PRINGSHEIM. 



fertilization results not only in 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 

 to the other. In Coleochaete it was 

 supposed that the sexual generation 



FIG. 84. Coleochaete scutata: dis- 

 coid form; the two larger and shaded 

 cells are developing as oogonia. 



