172 PHYSIOLOGICAL BOTANY. PART II. 



fold and open as usual, excepting that the periods 

 are somewhat lengthened ; on the other hand, when 

 they are exposed to a continued light, these periods are 

 shortened. When exposed to strong lamplight by 

 night, and excluded from all light by day, their periods 

 of sleep become extremely irregular for a time ; but, 

 in the end, the specimens generally close their leaves 

 during the day, and unfold them at night. The 

 alternate opening and closing of flowers is a similar 

 function to that of the sleep of leaves. The time of 

 day in which flowers close is very different for different 

 species, and even differs for that period during which 

 the leaves are asleep on the very same plant. Bertho- 

 let mentions an acacia in the garden at Orotava in 

 Teneriffe whose leaflets closed at sunset and unfolded 

 at sunrise, whilst its flowers closed at sunrise and 

 expanded at sunset. 



(I5f).) Electricity. Nothing very decisive is 

 known of the effects which so important an agent as 

 electricity produces on vegetation. It is, indeed, sup- 

 posed to act as a stimulant, and the supposition is 

 countenanced by the increased vigour with which plants 

 are observed to grow during the prevalence of stormy 

 weather. It seems to be not unlikely, that some 

 trees are more liable to be struck by lightning than 

 others ; but they are all so constructed as to present 

 numerous conducting points in the extremities of their 

 branches, well adapted for drawing off the electricity 

 in the clouds. 



(157-) Temperature. The influence of temper- 

 ature on vegetation is a very important consideration, 

 whether we regard the physical or physiological effects 

 which it produces. When the temperature is below 

 the freezing point plants can obtain no nutriment, be- 

 cause the water in which it is conveyed is solidified. 

 But further, it is essential to the healthy condition of 

 every plant that its internal temperature should be sup- 

 ported within certain limits, which differ for different 

 species. The opposite extremes of temperature under 



