THE GROWING PLANT 



51 



cuticle (64), the number of stomata (65), and so on. 

 Some plants, as purslane, the sedums and cacti, have 

 special water-storing tissues, from which transpiration is 

 extremely slow. 



Experiments indicate that the transpiration from most 

 leaves is between one-third and one-sixth as much as 

 the evaporation from an equal area of water. When 

 we take into account the immense leaf surface of a large 

 tree, it is evident that the aggregate transpiration must 

 be very great, as is often illustrated by the dwarfing in- 



FiG. 26. — Competition: showing how a spruce hedge dwarfs an adjacent 

 crop in dry weather. 



fluence of trees upon adjacent crops in dry weather 

 (Fig. 26). Transpiration is much more rapid during 

 dry than during wet weather, and in the rare atmos- 

 phere of high altitudes than in the denser atmosphere 

 of low lands. 



Excessive transpiration, as occurs in very dry weather, 

 is detrimental to plants, since it reduces the water pres- 

 sure within the cells below the point where healthful 

 ' growth can take place (62) ; but normal transpiration 

 i.e., not sufficient in amount to interfere with healthful 

 growth, is doubtless beneficial, since it aids in carrying 



