IRRITABILITY. 525 



readily the smaller the zoospores. This taking up of a definite 

 position by the organism with reference to the direction of the 

 incident rays Strasburger terms Phototaxis. Phototaxis has 

 also been observed in certain Desmids ; by Braun in Penium 

 curium, and by Stahl in a species of Pleurotaenium, in Closte- 

 riurn moniliforme, and others. These Desmids place them- 

 selves with their long axes parallel to the direction of the 

 incident rays when the light is moderately intense, and in 

 Penium and Pleurotaenium it is always the younger half 

 which is directed towards the source of light, but in Closterium 

 Stahl observed a periodical reversal of position such that, for 

 a time, one end is directed towards the source of light, and 

 then the body is swung over like a pendulum, so that the 

 other end comes to be directed towards the source of light. 

 When, however, the light is intense, these Desmids, according 

 to Stahl, place their long axes perpendicularly to the direction 

 of the incident rays. Another instance of phototaxis is afforded 

 by the changes of position of chlorophyll-corpuscles under the 

 influence of light, to which allusion was made in a previous 

 lecture (p. 299). Thus Frank and Stahl have observed that 

 when delicate structures like Fern-prothallia, Moss-leaves, or 

 Vaucheria filaments, are exposed to diffuse daylight falling 

 upon them in one direction only, the chlorophyll-corpuscles 

 collect on the cell-walls which are perpendicular to the direc- 

 tion of the incident rays (see Fig. 36, B, p. 300). Stahl has 

 also observed that the chlorophyll-band of Mesocarpus, when 

 the light is moderately intense, is perpendicular to the direc- 

 tion of the incident rays. 



With regard, now, to the intensity of the incident rays it 

 appears that zoospores move towards the source of light when 

 its intensity is below a certain degree, and move from it when 

 its intensity is above this critical degree. This critical inten- 

 sity is by no means the same in all cases, nor even in all 

 zoospores of the same kind, nor in the same zoospore at 

 different times. It depends, obviously, upon the sensitiveness 

 of the zoospore, and this is readily affected by various condi- 

 tions. It is affected, in the first place, by temperature. Thus, 

 supposing that at a certain medium temperature, zoospores 



