THE LEAF IN RELATION TO ITS ENVIRONMENT 293 



tions restricted : in dry 

 weather, on the con- 

 trary, the leaves are 

 rolled up so that the 

 leaf almost forms a tube, 

 the outer surface of 

 which is hard and quite 

 impervious to water." 

 The mechanism by which 

 this is effected is to be 

 found in certain cells, 

 which form longitudinal 

 rows at the base of the 

 furrows on the under 

 surface of the leaves, 

 and which are very sensi- 

 tive to moisture. In 

 damp weather the cells 

 increase in turgid ity by 

 absorption of water, and 

 the leaf opens. 



In concluding these 

 observations on the 

 dangers to plants from 

 excessive heat, and the 

 means provided by 

 Nature to counteract 

 those dangers, one is 

 naturally led to the op- 

 posite side of the sub- 

 ject, and the question 

 arises, If too much heat 

 be injurious to a plant, 

 may not, under contrary circumstances, too great loss of heat be injurious 

 too ? Moreover, if the all-wise Mother has devised means for protecting 

 growing plants from the one evil, may she not also have devised means 

 for protecting them from the other? 



To both questions an affirmative answer may be given. Loss of heat 

 has no less to be provided against than excessive transpiration the damp 

 and chilly nights must be taken into account quite as carefully as the dry 

 and sunny days ; for growth goes on in the plant more rapidly in some cases 

 by night than by day, and if it were systematically deprived of heat during 

 the hours of darkness, it would soon languish and die. Now the means 

 which Nature has devised for protecting growing plants from loss of heat 



FIG. 358. TELEGRAPH-PLANT (Desmodium gyrans). 



The small lateral leaflets move un and down, twisting at the same time, and 



describe a kini of ellipse. The movement i.s not continuous, and is much 



influenced by temperature. 



