32 



uniformly more sluggish in winter than in summer, in a cold 

 than in a warm room. 



" In the valisneria the motion ceases entirely at about 45°, 

 but the circulation will go on ii) the chara in water at 32°, 

 although slowly ; a greater degree of cold kills the plant and 

 destroys the motion. 



" The effect of an increased temperature is not so decided as 

 might be supposed; any heat above 150° destroys the circu- 

 lation entirely, apparently by changing the actual composition 

 of the cells and their contents ; but a more moderate applica- 

 tion has no uniform effect in increasing the rapidity of the 

 motion, although such has been affirmed to be the case by 

 other observers. The author has certainly found that by 

 raising the temperature, circulation has commenced in cells 

 where it did not exist before, and that in some others, where 

 it was slow, it has become faster ; but too much stress must 

 not be laid upon this, as the result is by no means uniform, 

 and the motion in many still remains slow ; it is, moreover, 

 very difficult ever to find again precisely the same cell for 

 differential observations. It may, however, be safely granted, 

 that there is a decided difference in the rapidity and generality 

 of motion at 40° and 70°. 



" The influence of light is exceedingly apocryphal. Meycn 

 has kept plants preserved months in a dark situation, care- 

 fully covered, at a temperature of 50°, and observed in them, 

 at the end of that time, motions quite as active as these plants 

 exliibit in summer, even at higher temperatures. 



" The author has repeatedly kept slips of the valisneria in 

 total darkness for many days, exposing them to bght no longer 

 than was necessary to renew the water they were placed in, but 

 has never found any effect produced on the circulation. He 

 has examined a portion taken when the plant was evolving 

 oxygen rapidly under the influence of strong sunshine, and 

 again when it has been deprived of bght altogether, but never 

 found any difference between the one and the other. 



