52 PRINCIPLES OF PLANT CULTURE 



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 greenhouses unless abun- 

 dant ventilation is given to encourage transpiration. 



75. Influence of trees. — Trees are detrimental to 

 crops in their vicinity not only by the shade they cause, 

 but also by their exhausting effect upon the soil moisture 

 in dry weather. The area afPected by a group of trees is 

 often much larger than is supposed. Fig. 26 shows how 

 an evergreen hedge may restrict the growth of corn in an 

 adjoining field. We should not infer from this, however, 

 that trees are on the whole detrimental to agriculture. 

 They serve many useful purposes. 



Experimental crops intended to be comparable with 

 each other should not be planted near growing trees. 



76. The brittleness of young plant tissues depends 

 upon the degree of water pressure within the cells. Foli- 

 age is usually most brittle during the morning and least 

 brittle during the latter part of the day, because tran- 

 spiration is most active during the warm hours of the 

 day. Lettuce and other salad plants are, therefore, 

 apt to be more crisp and tender when cut in the morning. 

 Tobacco, in which breaking of the leaves is harmful, is 

 preferably cut in the afternoon. Young hoed crops are 

 generally less injured by the smoothing harrow in the 

 afternoon than in the morning, and grass intended for 

 hay usually dries soonest when cut in the afternoon. 

 Lawn grass generally cuts easier in the morning than in 

 the afternoon. Slightly withered vegetables may have 

 their crispness partially restored by soaking them in 

 water for a time. 



77. The transpiration current. — Since the water of 

 plants is taken in from the soil through the root-hairs 



