How Substances are Transported and Removed 199 



no other possible source than as vapor from the leaves. This is 

 the source also of most of the moisture that collects upon win- 

 dows near which house plants are grown, and likewise of the 

 water-drops which gather, sometimes to annoying extent, on the 

 glass faces of ferneries, though such water is commonly assumed 

 to originate from evaporation out of the soil. This release of 

 vapor from leaves or other green parts is a practically universal 

 phenomenon in plants. It is called in physiology Transpiration; 

 and I wish to warn the reader at this point, out of the depths of 

 a considerable experience as a teacher, not to allow a mere re- 

 semblance in words to create any confusion in his mind between 

 this and the utterl}^ unrelated process of Respiration. Transpira- 

 tion is one of the great primal physiological facts about green 

 plants, and it has, like Photosynthesis, this further distinction, 

 that it is one of the very few processes of plants for which there 

 is no equivalent in animals, the animal process of perspiration 

 being utterly different both as to method and meaning. The 

 reader should therefore incorporate into the visualized picture 

 of the living plant now under construction in his imagination, 

 the idea of a tenuous cloud of vapor rising forever from all its 

 green parts. 



But no student of science, and therefore I hope not the reader, 

 will rest content with the general fact that water is given off as 

 vapor by plants, but will insist upon knowing the quantity. The 

 most practicable and accurate of the several methods by which 

 transpiration quantities may be determined lies in the use of the 

 balance. If one takes an ordinary potted plant, — Fuchsia, 

 Hydrangea, Rubber Plant, or other, — encloses soil and pot in a 

 water-tight cover to prevent evaporation therefrom, then weighs 

 the plant at intervals on an accurate balance, the comparative 

 weights, aside from some minor, and largely self-compensating, 

 errors arising from photosynthesis and respiration, must obvi- 

 ously exhibit the exact transpiration from the leaves and the 

 stems. Such experiments are frequently tried in botanical 



