430 VEGETABLE PHYSIOLOGY 
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 described 
is put out by the ccenocytic Alga Vaucheria. It appears 
as a mass of protoplasm, which becomes separated from 
the contents of a filament, and is set free by an opening at 
the apex of the latter. It is composed of several proto- 
plasts which are arranged together as in the rest of the 
ceenocyte, but their individual outlines cannot be seen. 
The fact that it is coonocytic 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 some- 
times called a Zooccenocyte. 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comes to rest, the cilia are withdrawn, and it becomes 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 
x another. On account of this 
WE ; ie yf peculiarity they are known 
d as sexual cells or gametes. 
In the lowliest forms, 
such as many filamentous 
Algew, they are produced by 
the same filament as the 
Fro. 169.— Part or a FILaMent oF asexual cells or gonidia. In 
Ulethrix FROM WHICH THE GamMETES the green Alga Ulothrix we 
g ARE ESCAPING Lea é 
Aitreicaueed peyoouene find the first indication of 
conjugating. these sexual cells. Besides 
the large zoogonidia with 
their four cilia, other smaller free-swimming bodies are 
developed in certain cells of the filament. They are pro- 
duced in larger numbers and have only two cilia each (fig. 
169). After they are set free into the water they swim 
