IRRITABILITY. 527 



surfaces are exposed to the light : when the light is intense, 

 they are collected on the cell-walls which are parallel to the 

 direction of the incident rays (light-apostrophe), so that they 

 present their edges to the incident rays. Stahl has observed 

 much the same thing in the case of the chlorophyll-band 

 of Mesocarpus. When the light is only moderately intense, 

 the chlorophyll-band presents its flat surface to the incident 

 rays, as mentioned above; when the light is intense, it 

 presents its edge to the incident rays. 



Phenomena of this kind have also been observed in the 

 case of the plasmodia of Myxomycetes. All observers 

 agree that the plasmodia move away from intense light, but 

 it is not clear whether, under any circumstances they move 

 towards the light. Baranetzky asserts, with reference to the 

 plasmodia of Aethalium and Didymium, that they always 

 move away from the light, and the more markedly so the 

 more intense the light, an assertion which is confirmed by 

 Stahl, although Hofmeister states that rather fluid plasmodia 

 move towards light at least in certain stages of their develop- 

 ment. Strasburger mentions some experiments in which 

 plasmodia of Aethalium came to the surface of a mass of 

 spent tan in faint light ; but there is no ground for concluding 

 that, in these experiments, the faint light determined the 

 direction of the movement of the plasmodia, for Aethalium- 

 plasmodia come readily to the surface of tan in complete 

 darkness. 



With regard to the distribution of phototactic influence 

 in the spectrum, it appears, from the observations of Cohn 

 and of Strasburger on zoospores, from those of Baranetzky 

 on plasmodia, and from those of Sachs on chlorophyll-cor- 

 puscles, that this influence is confined to the rays of high 

 refrangibility. 



The influence of other external agents in determining 

 the direction of movement has been chiefly studied in the case 

 of the plasmodia of Myxomycetes. Hofmeister and Rosanoff 

 appear to have been the first to investigate the subject, and 

 they concluded from their observations that the plasmodia 

 of Aethalium septicum are negatively geotropic. This view 



