Thraustotheca , a Peculiar Water-Mould. 163 
nutrient solutions, or even non-nutrient solutions which increase the osmotic 
pressure, are allowed to diffuse into the water, encystment immediately 
occurs. The zoospore becomes quiescent, and contracts to a sphere, while 
the contractile vacuole disappears and large granules appear in the finely 
granular content. Usually the cilia, still bending actively into twisted loops 
(Fig. 19), are dropped off during contraction of the zoospore ; but occasionally 
they seem to deliquesce into minute droplets and disappear. 
The cystospore thus formed is, in contrast to the sporangiospore, 
almost exactly spherical in shape, about 10/a in diameter, and possesses a 
more sharply defined wall, enclosing a denser content. Germination, which 
Text-fig. 2. Zoospores. Diagrams showing the characteristic shape. From camera lucida 
drawings of living spores. 1. Side view, the active anterior cilium at the left, the trailing posterior 
cilium at the right. The groove (above) cannot be seen. 2. From above, looking down on the 
groove, anterior cilium at the left. 3. Oblique, posterior-lateral view. The groove is partly shown. 
4. Posterior view showing groove and position of cilia. 
may occur at once or after some weeks of inactivity, is invariably by the 
formation of a hypha (Fig. 20). In hanging-drop cultures the growth 
of this hypha into a bit of fly muscle, the formation of rhizoids, and the 
ultimate development of a new plant can easily be observed. Since very 
little food material is present, the resulting plants are small (about 300- 
400 /x in length), and bear sporangia containing only a few spores (Fig. 22). 
Occasionally, also, the hypha of germination jnay form a dwarf sporangium 
(Fig. 23) similar to those which arise from the sporangiospores, and will be 
described later. 
The sporangiospores form hyphae of germination in either solid 
or liquid media. Rapid germination and vigorous growth to a mycelium 
occur when sporangiospores are transferred to beef decoctions, cornmeal 
mush, and cylinders of potato or carrot. Under these conditions the spore 
becomes vacuolate, swells rapidly (Fig. 24), and sends out a tube (Fig. 25) 
M 2 
