CHAPTER XIV. 



TRANSPIRATION: THE TRANSPIRATION- 

 CURRENT. 



Transpiration. — If the leaf of a growing sunflower or Jerusalem 

 artichoke is enclosed on a warm bright day in a wide test-tube 



as in Fig. .85, and the end of 

 the tube closed with a split 

 cork (f) or a plug of cotton- 

 wool, it will be noticed that 

 the inside of the tube soon 

 becomes covered with a dew- 

 like film of pure water which 

 gradually trickles down and 

 coUectsin considerable amount 

 as indicated at a. 



From all parts of ordinary 

 land plants there is going on 



Fig. 85. 



a continuous invisible loss of water in the form of vapour, and 

 unless precautions are taken to collect the water in some manner 

 similar to that described above the existence of its escape from 

 plants into the air is not easily realized. 



The exhalation of water in the form of vapour from living 

 plants is termed transpiration : it is not a mere physical process 

 of evaporation or drying such as occurs when a damp towel is 

 exposed to the air, but is a physiological process, which, although 

 influenced by external conditions, is nevertheless controlled to 

 some extent by the living protoplasm of the plant. Dead 



