PHYSIOLOGY 



471 



lost from the stem and branches, it is the leaves which are the prin- 

 cipal transpiring organs, and their size and number are regulated by 

 the air conditions to which the plant is exposed. The reduction of 

 the evaporating surface, or the checking of evaporation by the de- 

 velopment of a thick cuticle, or other protective devices, characteristic 

 of xerophytic plants, are associated with the checking of evaporation. 

 So perfectly is this accomplished in such plants as a Cactus, for 

 example, that a cut branch may lie exposed to the hot sun for weeks 

 without losing all its water. 



Sometimes in 

 hot weather tran- 

 spiration is so 

 active that for a 

 time the absorp- 

 tion of water by 

 the roots is insuf- 

 ficient to supply 

 the loss of water 

 by transpiration, 

 and the plant be- 

 comes wilted. So 

 soon as the evapo- 

 ration is 'checked, 

 as is the case at 

 night, the wilted 

 organs become tur- 

 gid again. 



The temperature 

 of the soil is of 

 importance in the 

 absorption of 

 water by the root- 

 hairs, which 

 become inactive 

 below a certain 

 temperature. Thus 

 the Tobacco plant, 

 even when there 

 is abundant mois- 

 ture in the soil, 

 wilts if the soil- 

 temperature falls 

 below 2-4 C. 



FIG. 452. A, apparatus to show lifting force of a transpiring 

 branch, indicated by the column of mercury in the tube, 

 the upper part being filled with water into which the cut 

 end of the branch descends. B, Apparatus showing a 

 wilted branch a, restored to its normal condition, b, by 

 water forced into it by the pressure of the column of mer- 

 cury, m, m l . (A, after MACDOUGAL ; B, after SACHS.) 



The amount of water which is lost by transpiration may be very 

 considerable. A strong Sunflower plant, on a warm day, will lose 



