58 Principles of Plant Culture. 



dennis and cuticle (65), the number of stomata (66) etc. 

 Some plants, as purslane, the sedums, cacti etc., have 

 special water-storing tissue, 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- 

 fluence of trees upon adjacent crops in dry weather (Fig. 

 26). Transpiration is much more rapid during dry 



FIG. 26. Showing how a spruce hedge dwarfs an adjacent corn crop in 

 dry weather. 



than during w,et weather, and in the rare atmosphere 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 (63); but normal transpiration, 

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

 growth, is doubtless beneficial, since it aids in carrying 



