324 VEGETABLE PHYSIOLOGY 



When we review the phenomena of transpiration we 

 find two very important considerations presenting them- 

 selves to us. On the one hand, the suggestion comes that 

 the enormous stream of water passing through a terrestrial 

 plant is necessary in order that a sufficient amount of 

 inorganic salts may be supplied to the leaves, and that the 

 process of transpiration is maintained so that such a supply 

 may be at the disposal of the protoplasts. The dilute 

 solutions which are absorbed naturally involve the trans- 

 port of a large amount of water with the salts. Transpira- 

 tion seems thus to be subordinate to food supply. 



On the other hand, the temperature relations which we 

 have just examined appear to place transpiration upon 

 quite a different plane. Instead of being a subordinate 

 process, it appears to be imperative in order to prevent a 

 fatal rise of temperature in the metabolic protoplasts ; to 

 be concerned primarily, that is, in the regulation of the 

 conditions necessary for the maintenance of metabolism 

 and life, rather than in the supply of material for metabolic 

 purposes. 



Which of these is the chief function of transpiration 

 will probably depend upon circumstances. The process 

 serves the two purposes, sometimes one, sometimes the 

 other, being the more prominent. 



Another cause of loss of heat is found in radiation, 

 which takes place to a very important extent from the 

 surfaces of flattened organs such as leaves. This radiation 

 is to a certain extent independent of the temperature of the 

 surrounding air, and leads in some cases to a leaf being 

 several degrees cooler than the latter. A thermometer 

 placed on the grass will frequently show a temperature 

 some nine or ten degrees lower than another one suspended 

 a few inches above the surface of the ground. Evidence 

 of the activity of radiation at night is afforded by the 

 constant appearance of dew or hoar-frost on the leaves. 



The effects of excessive radiation often threaten to be 

 disastrous and have led to the development of many pro- 



