VEGETATIVE REPRODUCTION 207 



The most common method by which the 

 simpler aquatic algae 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, i. e. protoplasts 

 which remain enclosed in cellulose membranes, 

 replace the naked zoospore. From this simple 

 stage the rest is easy. A few coherent cells 



