CHAPTER VI. 



TRANSPIRATION. 



70. We should now inquire if all the water which is taken up 

 in excess of that which actually suffices for turgjdity is used in the 

 elaboration of new materials of construction. We notice when a 

 leaf or shoot is cut away from a plant, unless it is kept in quite 

 a moist condition, or in a damp, cool place, that it becomes flac- 

 cid, and droops. It wilts, as we say. The leaves and shoot 

 lose their turgidity. This fact suggests that there has been a 

 loss of water from the shoot or leaf. It can be readily seen that 

 this loss is not in the form of drops of water which issue from the 

 cut end of the shoot or petiole. What then becomes of the water 

 in the cut leaf or shoot ? 



71. Loss of water from excised leaves. Let us take a hand- 

 ful of fresh, green, rather succulent leaves, which are free from 

 water on the surface, and place them under a glass bell jar, which 

 is tightly closed below but which contains no water. Now we 

 will place this in a brightly lighted window, or in sunlight. In 

 the course of fifteen to thirty minutes we notice that a thin film 

 of moisture is accumulating on the inner surface of the glass jar. 

 After an hour or more the moisture has accumulated so that it 

 appears in the form of small drops of condensed water. We 

 should set up at the same time a bell jar in exactly the same way 

 but which contains no leaves. In this jar there will be no con- 

 densed moisture on the inner surface. We thus are justified in 

 concluding that the moisture in the former jar comes from the 

 leaves. Since there is no visible water on the surfaces of the 

 leaves, or at the cut ends, before it may have condensed there, 



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