644 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



ovoid or spherical, and usually possess a lateral beak. While the antheridium pro- 

 duces a large number of the small spermatozoids, the oogonium gives rise to a 

 single large egg. An account has already been given of the process of fertilization 

 (see p. 58, and figs. 204 ^ and 204 ^ on p. 53). 



Vaucheria also produces peculiar zoospores, whose development and subsequent 

 behaviour have been described on pp. 23 and 24 of vol. i. (see Plate I. a-d). The 

 cilia with which the surface of the zoospore is clothed are arranged in pairs, and in 

 the colourless external layer of protoplasm just below each pair of cilia is situated a 

 single nucleus. This suggests that the zoospore of Vaucheria is to be regarded as 

 really equivalent to a great many zoospores which have not separated during 

 development, each nucleus, with its pair of cilia and a certain amount of chloro- 

 phyll and protoplasm, representing an ordinary zoospore. It has been found that 

 if, as often happens, the zoospore breaks into two during its struggles to escape 

 from the end of its tube, the front part rounds itself off and swims away, behaving 

 just like an entire zoospore. In some species of Vaucheria the zoospores are 

 only partially clothed with cilia and come to rest soon after their escape. In yet 

 others they have no cilia at all, and either escape by dissolution of the end of the 

 tube, or germinate in situ. Here then we have a transition from the formation 

 of active zoospores to the production of passive aplanospores. 



The occurrence and form of reproduction is here even more entirely under the 

 control of conditions than is the case in Hydrodictyon. The age and size of the 

 plant are no longer factors, since sexual organs and zoospores can be formed on 

 quite short germ-tubes. Cultivation of the plant in a solution rich in inorganic 

 food-salts always gives it a tendency to produce zoospores, but the immediate 

 stimulus to their formation is given by a distinct change in the conditions, just as 

 is the case in the Water-net. In Vaucheria this change is especially necessary, 

 since each zoospore is formed in the apex of a tube, and apical growth must be 

 stopped in order to allow free play to the zoospore-forming forces. 



The nature of the change, so far as regards the medium, is apparently im- 

 material — it may be a change from running water to still water, or from a damp 

 atmosphere to water, or in the temperature or concentration of the culture-solution, 

 but it is a change from light to darkness which is especially effective. This is quite 

 contrary to the case of Hydrodictyon. The presence of water and a temperature 

 between 3° and 26° C. are absolutely necessary conditions of zoospore-formation. ; 



The former condition is obviously adaptive. The formation of sexual organs 

 is specially induced by the replacement of inorganic salts by organic substances (e.g. | 

 sugar), just as in the case of the gametes of Hydrodictyon, but antheridia and j 

 oogonia, which are formed much more frequently and easily than are the Water-net t 

 gametes, often appear in the presence of salts, and when growing in a damp atmos- 

 phere. Light and a temperature above 3° C. are absolutely necessary conditions. 



The most striking point in the physiology of the reproduction of Vaucheria is i 

 the prominent part played by the sexual organs. These are undoubtedly the princi- ■ 

 pal means of propagation the plant possesses, the zoospores, which usually fulfil this 



