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- 
