278 VEGETABLE PHYSIOLOGY 
phorescence. Heat is lost to the plant in many ways, one 
of which, the evaporation of the water of transpiration, has 
already been mentioned. Another almost equally important 
source of loss is radiation from the general surface. This 
is greatest from flattened members of the plant, such as 
leaves. 
The temperature of the plant is very largely influenced 
by that of the air, and no doubt interchanges of heat take 
place in both directions. But it must not be concluded that 
the temperature of the plant and that of the air always 
vary together. On the contrary, radiation under some 
conditions may go on until the plant is several degrees 
colder than the surrounding air. This is probably the 
explanation of the ready formation of dew and hoar frost 
on the surfaces of leaves at certain seasons of the year. 
It is quite of frequent occurrence again that a plant or part 
of a plant may have a much higher temperature than the 
air, and hence a copious radiation may take place. During 
the processes of germination the temperature of the seed 
may be as much as 20° C. above that of the air. The 
opening of flower buds is also attended by the attainment 
of a high temperature and a consequent escape of heat. 
If we turn again to plants with a watery environment, 
the loss of heat may be observed under appropriate con- 
ditions. It is well known that the processes of alcoholic 
fermentation provoked by the yeast-plant are attended by 
the liberation of heat, which is given off by the active 
cells, and causes a considerable rise of temperature in the 
fermenting liquid. 
We may infer also from a consideration of the various 
processes we have studied, and from the fact that they are 
carried out most advantageously within a certain relatively 
small range of temperature, that the maintenance of such 
a temperature is a great desideratum to the plant. There 
is not a very complete mechanism in the plant to secure this 
object, for the organism generally becomes of about the 
same temperature as the medium in which it grows, though 
