VEGETATIVE REPRODUCTION 207 
The most common method by which the 
simpler aquatic alge reproduce themselves 
vegetatively is by giving birth to zoospores. 
The protoplasmic contents of a cell contract 
away from the wall, cilia are developed, and 
the zoospore escapes through a hole which is 
formed in the cell wall. Very often a series 
of adjacent cells may be seen all to give rise 
to zoospores in this way. Sometimes the 
zoospores are not so simple, and represent not 
single cells only but a cluster, the individuals 
of which are not delimited by walls from 
each other. The huge zoospore of Vaucheria 
belongs to this type; it is easily visible to the 
naked eye, as it rolls about through the water 
by means of its numerous pairs of cilia. 
But however the zoospores are formed, 
they generally settle down after a period 
of independent movement. They withdraw 
their cilia, secrete a cell wall over their naked 
surface, and grow into an organism generally 
similar to that from which they themselves 
have sprung. 
It is different with land plants. Motile 
propagative bodies would be _ practically 
useless here, and the nakedness of the proto- 
plasm would render them specially susceptible 
to numerous adverse influences inseparable 
from existence on land. In the simpler forms 
we find that entire cells, 7.e. protoplasts 
which remain enclosed in cellulose membranes, 
replace the naked zoospore. From this simple 
stage the rest is easy. A few coherent cells 
