330 VEGETABLE PHYSIOLOGY 



is noteworthy that whether the leaf is brightly or only 

 moderately illuminated the same relative proportions of the 

 total energy absorbed are devoted to the purposes of trans- 

 piration and photosynthesis. 



When we review the phenomena of transpiration we hnd 

 two very important considerations presenting themselves 

 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 in- 

 organic 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 solu- 

 tions which are absorbed naturally involve the transport 

 of a large amount of water with the salts. Transpiration 

 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 sur- 

 rounding 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 



