THALLOPHYTA sll 
well-developed nucleus; in these features they are much more 
advanced than the blue-green alge. Propagation is by means 
of the division of a cell (Fig. 319), no sexual reproduction 
being known. 
The most primitive of the green alge are somewhat more ad- 
vanced types of unicellular plants, which, like the flagellates, 
move by means of flagella (Fig. 320). In these there are two 
types of reproduction. The cell contents may divide (Fig. 322) to 
form a number of small individuals, or zodspores, which are like 
mature individuals except in size, 
and which, without sexual fusion, 
grow into mature individuals; 
or the cell contents may divide 
into a somewhat larger number of 
smaller bodies, or gametes, which 
have the same structure as z06- 
spores, but which fuse in pairs 
to give rise to new individuals 
(Fig. 323). Reproduction by fu- 
sion of two similar gametes is a 
very simple form of sexuality. A single-celled individual and colo- 
esa niewhat higher types me nies of two, three, and four cells. 
(x 1285) 
plants sexual reproduction may be 
due either to the fusion of similar gametes or to the fusion of a 
large and a small gamete (Fig. 328), while in still more advanced 
types the large gametes become nonmotile eggs which are ferti- 
lized by small gametes called spermatozoids (Figs. 331, 333). 
Just as the algze show an evolution in the development of 
sex, they also exhibit a gradual development of a nonmotile 
vegetative body. One of the most primitive indications of a 
nonmotile vegetative body is found in certain primitive, motile, 
unicellular forms of green alge. In these, under certain condi- 
tions, the individuals may withdraw their flagella and by divi- 
sion give rise to a colony of nonmotile cells held together by a 
gelatinous matrix (Figs. 320, 821). When conditions again be- 
come fayorable for the motile state, the cells send out flagella 

Fig. 312. Gloeocapsa 
