TRANSPIRATION FROM PLANTS 177 
Second, transpiration increases with the increase 
in temperature; that is, under conditions otherwise 
the same, transpiration is more rapid on a warm day 
than on a cold one. The temperature increase of it- 
self, however, is not sufficient to cause transpiration. 
Third, transpiration increases with the increase of 
air currents, which is to say, that on a windy day 
transpiration is much more rapid than on a quiet day. 
Fourth, transpiration increases with the increase 
of direct sunlight. It is an interesting observation 
that even with the same relative humidity, tempera- 
ture, and wind, transpiration is reduced toa minimum 
during the night and increases manyfold during the 
day when direct sunlight is available. This condi- 
tion is again to be noted by the dry-farmer, for the 
dry-farm districts are characterized by an abundance 
of sunshine. 
Fifth, transpiration is decreased by the presence 
in the soil-water of large quantities of the substances 
which the plant needs for its food material. This 
will be discussed more fully in the next section. 
Sixth, any mechanical vibration of the plant 
seems to have some effect upon the transpiration. 
At times it is increased and at times it is decreased by 
such mechanical disturbance. 
Seventh, transpiration varies also with the age of 
the plant. In the young plant it is comparatively 
small. Just before blooming it is very much larger 
and in time of bloom it is the largest in the history of 
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