REPRODUCTION 491 
duce spores when conditions are suitable for the life of 
the ordinary individual, but can be made to do so by 
cultivating them under adverse conditions of moisture, 
food supply, &c. 
A somewhat similar structure to the zoogonidia de- 
scribed is put out by the ceenocytic Alga Vaucheria. It 
appears as a mass of protoplasm, which becomes separated 
from the contents of a filament, and is set free by an open- 
ing at the apex of the latter. It is composed of several 
protoplasts which are arranged together as in the rest of 
the coenocyte, but their individual outlines cannot be seen. 
The fact that it 1s ccenocytic is shown by the presence of a 
number of nuclei in the protoplasmic mass. A pair of 
cilia are given off opposite to each nucleus, so that it 
swims very readily in the water after its liberation. It 
is sometimes called a Zoocenocyte. After a period of 
motility it comes to rest, the cilia are withdrawn, and it 
becomes clothed by a cell-wall. The resting period lasts 
for a variable time, after which it develops into a new 
Vaucheria filament. 
Besides these asexual reproductive bodies other cells 
are produced by the great majority of plants, which are 
incapable of giving rise to new individuals, unless two of 
them unite or fuse with one another. On account of this 
peculiarity they are known 
as sexual cells or gametes. 
In the lowliest forms, 
such as many filamentous 
Alge, they are produced by 
the same filament as the 
asexual célls or gonidia. In 
the case of Ulothria we Fic. 169.—Part oF A FILAMENT oF 
find the first indication of & Pee aaa a eal 
these sexual cells. Besides g', free gamete; 9°, 9°, gametes 
the large zoogonidia with Panay 
their four cilia, other smaller free-swimming bodies are 
developed in certain cells of the filament. They are 
