TKANSPIRATION 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 sunhght. It is an interesting observation 

 that even with the same relative humidity, tempera- 

 ture, and wind, transpiration is reduced to a 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 jjlant 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 



