298 LECTURE XIV. 



movements of the plasmodium of Didymium Serpula were 

 arrested when the temperature was raised to 30 C., and that 

 they also ceased at a low temperature. With regard to 

 ciliary movement, Strasburger has found that the zoospores 

 of Hcematococcus lacustris tend to come to rest at from 

 8 4 C., but he has observed them in movement even in 

 water containing ice ; he fixes the optimum temperature for 

 these zoospores at 30 40 C., and the maximum at 50 C. 

 He also observed that the zoospores of Botrydium ceased to 

 move when the temperature sank to 6 C. The minima and 

 maxima for the zoospores of marine Algae are lower than 

 those for fresh-water Algae. Kjellmann observed the forma- 

 tion of zoospores going on in the Algse of the coast of Spitz- 

 bergen when the temperature of the sea was r5 C. to 

 r8C. : and Strasburger found that the zoospores of the 

 marine Algae which he observed were killed at 35 40 C. 

 Finally, with regard to the movements of organs in the higher 

 plants, Kabsch observed that the spontaneous movements of 

 the lateral leaflets of the Telegraph-plant \Desmodium (Hedy- 

 sarum) gyrans\ ceased when the temperature sank to 22 C., 

 that they went on slowly at 28 30 C. (a complete up and 

 down movement in 4 minutes), and that they were more rapid 

 at 35 C. (a complete movement in 85 90 seconds) : in the 

 case of the Sensitive Plant (Mimosa pudicd} Sachs observed 

 that if the plant be kept for some hours at a temperature 

 of about 1 5 C., its leaves gradually lose their motility ; the 

 lower the temperature below this degree the more rapidly 

 does the power of movement disappear; he found also that 

 the power of movement is lost within an hour when the plant 

 is kept in damp air at 40 C., within half an hour in air at 

 45 C., and in a few minutes in air at 49 50 C. 



Again, movement, like growth, is affected by light. In 

 some cases light appears to promote movement, in others to 

 arrest it. Thus Engelmann has discovered a form of Bac- 

 terium, termed by him Bacterium photometricum, which is 

 only motile when exposed to light ; and Sachs has found 

 that if plants with motile leaves (Mimosa, Acacia, Trifolium, 

 Phaseolus, Oxalis) are kept for some days in darkness, or even 



