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 find 
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 ig 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 bemg a subordinate 
process, it appears to be imperative m 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 ig found in radiation, which 
takes place to a very important extent from the surfaces 
of flattened organs such ag leaves. This radiation is to a 
certain extent independent of the temperature of the sur- 
rounding air, and leads in some cages 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 
