The Water of Plants and its Movements. 59 



food materials from the soil into the leaves (59). For 

 this reason, plants native to regions having a rather dry 

 atmosphere, do not thrive in greenhouses unless abund- 

 ant ventilation is given to encourage transpiration. 



76. Trees are Detrimental to Crops in their vicinity 

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

 exhausting effect upon the soil moisture in dry weather. 

 The area affected by a group of trees is often much 

 larger than is supposed. The illustration on page 58 

 (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 

 ^ach other should not be planted near growing trees. 



77. The Brittleness of Young Plant Tissues depends 

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

 liage is usually most brittle during the morning and least 

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

 piration is most active during the warm hours of the 

 day. Lettuce and other salad plants are, therefore, apt 

 to be most crisp and tender when cut in the morning. 

 Tobacco, in which breaking of the leaves is detrimental, 

 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. 



