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 



