THE GKEAT GROUPS OF ALG.E 
261 
fore, two sorts of asexual spores are produced: (1) the 
tetraspores, developed in ordinary sporangia; and (2) the 
carpospores, developed in the cystocarp, which has been 
produced as the result of fertilization. 
OTHER CHLOROPHYLL-CONTAINING THALLOPHYTES 
175. Diatoms. These are peculiar one-celled forms, which 
occur in very great abundance in fresh and salt waters. 
FIG. 230. A group of Diatoms : c and d, top and side views of the same form; e, colony 
of stalked forms attached to an alga;/ and g, top and side views of the form shown 
at e; A, a colony; i, a colony, the top and side view shown at k. After KERNER. 
They are either free-swimming or attached by gelatinous 
stalks ; solitary, or connected in bands or chains, or im- 
bedded in gelatinous tubes or masses. In form they are 
rod-shaped, boat-shaped, elliptical, wedge-shaped, straight 
or curved (Fig. 230). 
