526 LECTURE XX. 



move towards light of a given intensity ; if the temperature 

 be lowered several degrees, the intensity of the light remaining 

 the same, the zoospores will now move away from the source 

 of light, that is, their sensitiveness to light is now greater. 

 Again, supposing that at a certain medium temperature zoos- 

 pores move away from light of a given, and considerable, inten- 

 sity ; if now the temperature be raised several degrees, the 

 zoospores will move towards the source of light, that is, at the 

 higher temperature they are less sensitive to light. 



Strasburger gives, amongst others, the following illustration of the 

 relation of sensitiveness to temperature in the case of the zoospores of 

 Hcematocccus lacustris ; the light was throughout of the same intensity. 



16 1 8 C zoospores moved towards light, 

 4 C. - away from light. 



It may be mentioned here that the distribution of the zoospores 

 in a vessel of water is affected, as Sachs has shewn, by differences of 

 temperature in different parts of the liquid, which give rise to currents 

 which are strong enough to carry the zoospores along. Sachs was able, 

 with emulsions of oil, etc., to reproduce many of the phenomena pre- 

 sented by zoospores. It is quite clear, however, that the distribution 

 of the zoospores under the influence of light, is to be attributed only 

 in a small degree to these currents in the water ; it is mainly due to 

 their own sensitiveness to light. 



In the second place, the sensitiveness of zoospores to light 

 is affected by the supply of free oxygen. Strasburger observed 

 that when zoospores are kept in a closed vessel for some hours 

 their sensitiveness diminishes, that is, that at the end of the 

 time they move towards light from which, at the beginning, 

 they moved away. Finally, it appears that the sensitiveness 

 of zoospores increases with their age ; that when they are 

 young, they move towards light, from which, when they are 

 older, they move away. 



The position of the chlorophyll-bodies in the cells of 

 plants is also affected by the intensity of the light falling 

 upon them. Under the influence of light of moderate in- 

 tensity, the chlorophyll-corpuscles are collected on the cell- 

 walls which are perpendicular to the direction of incident 

 rays (epistrophe, see p. 300, Fig. 36, B), so that their flat 



