1 12 Upon the rate of growth by Plants 



cases of unusually rapid growth 7 took place in cloudy weather, 

 and only 3 in clear weather ; in the Willow, out of 7 such ex- 

 tremes 5 were in cloudy and only 2 in clear weather ; and in the 

 Passionflower 3 extremely fast growths took place in cloudy and 2 

 in clear weather. The Fig is however altogether an exception to 

 this supposed rule, for in 10 out 15 extreme cases it grew fastest 

 under bright light. Possibly this discrepancy may be accounted 

 for by the different nature of the plants under experiment. The 

 Willow, Passionflower and Vine are plants with a very thin skin, 

 and therefore will suffer considerable loss of their fluids, by evapo- 

 ration under bright light, which must obstruct their growth ; the 

 Fig on the other hand, being a plant with a peculiarly thick skin, 

 will suffer much less from this cause, and may indeed demand a 

 much larger supply of light than the others in order to perform its 

 functions in the most efficient way. 



But if the experiments were to a certain degree to confirm the 

 general opinion that plants grow fastest in warm cloudy weather, 

 it is also clear that they indicate the presence of other agencies 

 than light and heat, and a regular supply of moisture. The nu- 

 merous exceptions that are found even in those plants which in 

 rapid growth conform the best to the supposed rule show this 

 sufficiently well, and when we attempt to reduce to it the slowest 

 growths we fail entirely ; the facts inclining sometimes one way and 

 sometimes the other. 



One of the most singular facts brought out by these observations 

 is the total want of correspondence between the effects produced 

 upon plants by the same external circumstances. The subjects of 

 experiment were placed within a few feet of each other, in a house 

 heated very uniformly, and equally exposed to light, and to every 

 other agent by which it is conceivable that plants should be 

 affected. Yet strange to say, it appears certain that the same 

 causes do not produce the same results when operating upon plants 

 of different species. For example the greatest growth made by 



