86 Chapters in Modern Botany CHAP. 



inheritance differs much in different species, and seems 

 never to be very rigid; for plants have been introduced 

 from all parts of the world into our gardens and green- 

 houses ; and if their movements had been at all strictly 

 fixed in relation to the alternations of day and night, they 

 would have slept in this country at very different hours, 

 which is not the case ; moreover, it has been observed 

 that sleeping plants in their native homes change their 

 times of sleep with the changing seasons." 



The movements Darwin explained in two ways : " firstly, 

 by alternately increased growth on the opposite sides of 

 the leaves, preceded by increased turgescence of the cells ; 

 and secondly, by means of a pulvinus or aggregate of 

 small cells, generally destitute of chlorophyll, which be- 

 comes alternately more turgescent on nearly opposite sides ; 

 and this turgescence is not followed by growth except during 

 the early age of the plant." The movement may be 

 almost the same whether a pulvinus be present or not, 

 but in the former case the course of the leaves is more 

 regularly elliptical, and the movements are continued for a 

 much longer period in the life of the plant. 



The position occupied by the leaves at night indicates 

 that the benefit they derive is " the protection of their 

 upper surfaces from radiation into the open sky ; and 

 in many cases the mutual protection of all the parts from 

 cold by their being brought into close approximation." In 

 evidence of this Darwin showed that leaves compelled to 

 remain extended horizontally at night, suffered much more 

 from radiation than those which were allowed to assume 

 their normal vertical position. 



"Any one who had never observed continuously a 

 sleeping plant, would naturally suppose that the leaves 

 moved only in the evening when going to sleep, and in 

 the morning when awaking; but he would be quite mis- 



