164 THE AMERICAN BOTANIST 



is directed downward, wliile the lower surface is fully expos- 

 ed to the sky, and an arched roof thus foruied over the two 

 lateral leaflets which have their upper surfaces pressed closely 

 together. Here there is an unusual case where one of the 

 leaflets does not stand vertically at night. The position which 

 leaves occupy at night indicates with sufficient clearness, that 

 the benefit derived is the protection of their upper surfaces 

 from radiation into the open sky and in many cases the mutual 

 protection of all parts from cold, by their being brought into 

 closer approximation. 



The nyctitropic movements of leaves are easily affected 

 by conditions to which plants have been subjected. If the 

 soil be too dry, or if the humidity be very low, the movements 

 are much delayed, or fail altogether. Some have advocated 

 the idea that the quantity of water absorbed has a great in- 

 fluence on the movement of leaves and that the variable 

 amount of shrinking of leaves bears a relation to the amount 

 of water in the plant. It is the accepted belief however, that 

 the nyctitropic movements of leaves, leaflets and petfoles are 

 effected in two different ways : first, by the alternately increas- 

 ed growth on their opposite sides, preceded by increased 

 turgescence of the cells, and second by means of a pulvinus or 

 aggregation of small cells generally destitute of chlorophyl, 

 which also become alternately turgescent on nearly opposite 

 sides. When pulvini are present, the nyctitropic movements 

 are continued for a very much longer period than when such 

 do not exist. 



The sleep of cotyledons is as pronounced as the sleep of 

 leaves. As a general observation, 'cotyledons provided with 

 pulvini continue to rise or sink at night during a much long- 

 er period than those destitute of this organ. In this latter 

 case the movement no doubt depends on alternately greater 



