The Water of Plants and Its Movements. 57 
76. Trees are Detrimental to Crops in their vicinity 
not only by the shade they cause, but by their exhausting 
effect upon the soil moisture in dry weather. The distance 
affected by a group of trees is often much greater than is 
supposed. The accompanying illustration (Fig. 24) 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 generally detrimental to agriculture. Thev 
serve many useful purposes. 
Experimental crops intended to be comparable with each 
other sbould not be planted near growing trees. 
77. The Brittleness of Plant Tissues depends upon 
the degree of water pressure within the cells. Foliage is 
usually most brittle during the morning, and least brittle 
during the latter part of the day, because transpiration is 
most active during the warmer hours of the day. Lettuce 
and other salad plants are, therefore, apt to be most crisp 
and tender when cut in the morning. Tobacco, on the other 
hand, in which breaking of the leaves is detrimental, is pre- 
ferably cut in the afternoon. Young hoed crops are gener- 
ally less injared 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 gen- 
erally cuts easier in the morning than in the afternoon. 
Slightly withered vegetables may have their crispness 
partially restored by soaking in water for a time. 
78. The Evaporation Current. Since the water of 
plants is taken in from the soil through the root-hairs (101), 
and escapes more or less rapidly by transpiration (75), 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 
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