The Water of Plants and Its Movements. 59 



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 



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

 crop in dry weather. 



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 pressure 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 food materials from the soil into the 

 leaves (58). For this reason, plants native to regions 

 having a rather dry atmosphere, do not thrive in green 

 houses unless abundant ventilation is given to encour- 

 age transpiration. 



75. Trees are Detrimental to Crops in their vicinity 

 not only by the shade they cause, but also by their e*- 

 hausting effect upon the soil moisture in dry weather. 



