60 Principles of Plant Culture. 



The area affected by a group of trees is often much 

 larger than is supposed. Fig. 26 shows how an ever- 

 green hedge may restrict the growth of corn in an ad- 

 joining 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 de- 

 pends upon the degree of water pressure within the 

 cells. Foliage is usually most brittle during the morn- 

 ing and least brittle during the latter part of the day, 

 because transpiration 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 Evaporation Current. Since the water of 

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

 (100), and escapes more or less rapidly by transpiration 

 (74), it is clear that in leafy plants a current of water 

 must pass from the roots through the stem and branches 

 into the leaves, and that the rate of this current will 

 depend much upon the rate of transpiration from the 



