524 LECTURE XX. 



by Pringsheim with regard to the streaming movement of 

 protoplasm in the cells of Spirogyra, Nitella, and of the hairs 

 of Tradescantia, and by Hofmeister and Baranetzky in the 

 case of the plasmodia of Myxomycetes. 



These movements are also affected by other conditions. 

 They are affected, for instance, by the amount of water of 

 imbibition; thus the movements of zoospores are retarded by 

 increasing the concentration of the solution in which they are 

 swimming; in the case of the streaming movement of the pro- 

 toplasm there appears to be, according to Velten, a certain 

 optimum proportion of water at which the movements are 

 most active, any increase or diminution of which tends to 

 diminish the activity of the movement. Mechanical stimuli 

 also affect these movements; according to Strasburger the 

 movements of zoospores are temporarily arrested by vibration; 

 and pressure and concussion tend to arrest the streaming, as 

 well as the amoeboid, movement of protoplasm, and to cause 

 rounding-off of the protoplasm, an effect which is also produced 

 by electrical stimuli, though apparently in some cases a weak 

 stimulus gives rise to an acceleration of movement. The 

 movements are also affected by various chemical substances. 

 Strasburger found that zoospores are at once killed by mor- 

 phia, strychnia, curare, chloroform, salicylic acid, etc. The 

 streaming and the amoeboid movement of protoplasm is 

 arrested by chloroform and ether (Kiihne) ; and by dilute 

 alkalies (Dutrochet), but it is not clear that the latter is a 

 direct effect ; the alkali may operate by modifying the pro- 

 portion of water of imbibition. 



The direction of these different movements is influenced 

 in a striking manner by various external agents. Of the effects 

 thus produced, the most remarkable are perhaps those which 

 are due to the direction and intensity of the incident rays of 

 light. With regard to the influence of the direction of the 

 incident rays, it has been observed (Strasburger, Stahl) that 

 the zoospores of various plants (Ulothrix, Haematococcus, 

 Botrydium, Polyphagus Englena, Chytridium vorax) when 

 exposed to oblique illumination place their long axes parallel 

 to the direction of the incident rays, and this the more 



