IRRITABILITY. 531 



attracted. An alkaline solution, however weak, always re- 

 pelted. 



In concluding our study of these various forms of move- 

 ments and their relation to external agents we may briefly 

 consider their physiological significance, and compare them 

 with other phenomena with which we have already become 

 acquainted. In the case of motile organisms, their power of 

 locomotion and their irritability to external agents enable 

 them to seek a habitat in which the conditions of their life 

 are the most favourable. By means of zoospores the dis- 

 tribution of the plants producing them is ensured. The pro- 

 duction of motile and irritable antherozoids by plants of a 

 more or less aquatic habit is essential to ensure the fer- 

 tilisation of the oospheres ; in some such plants, such as the 

 Florideae, the antherozoids (spermatia) are neither motile nor 

 irritable, and in these the fertilisation of the female organ is 

 left entirely to chance. To this latter point we shall sub- 

 sequently recur. The streaming movement of the protoplasm 

 is doubtless connected with its nutrition, a view which is 

 supported by the fact that this movement can, in some cases, 

 only be detected when the formative processes (e.g. the forma- 

 tion of a wall after division) are most active. 



These movements which we are considering are spon- 

 taneous, given the necessary external conditions, and are 

 thus comparable to the spontaneous movement of growing 

 organs which we have already studied (p. 360). We see, 

 further, that these spontaneously moving masses of pro- 

 toplasm are affected by external agents much in the same 

 way as the spontaneously moving growing organs. We find, 

 for example, that the direction of movement of zoospores is 

 determined by the direction of the incident rays of light, just 

 as is the direction of growth of radial organs (p. 428). When 

 exposed to light, the zoospores place themselves so that their 

 long axes are parallel to the direction of the incident rays ; 

 this is precisely what radial organs tend to do under the 

 same circumstances. According to circumstances the zoo- 

 spores move either towards or away from the source of light ; 

 similarly, growing radial organs direct their apices towards or 



342 



