THALLOPHYTES 
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 Fi G -8 3 .Bulbochaete: 
. . . . . , sho wing a dwarf male fila- 
oogonia are free at the ends of branches, each ment> with its terminal 
developing a long tubular prolongation through antheridium, attached to 
which the sperm enters. an ogonium. After 
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 
FlG. 84. Coleochaete scviata: dis~ 
coidform; the two larger and shaded to the other ' - In Coleochaete it was 
cells are developing as oogonia. supposed that the sexual generation 
