TEMPERATURE AND ITS CONDITIONS 381 



of radiation at night is afforded by the frequent 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- 

 tective adaptations by various plants. The masses of 

 woolly hairs which are often found upon leaves, forming, 

 indeed, in some cases a thick mantle, must generally be 

 looked upon as such a defensive mechanism. The delicate 

 leaves of buds are often protected by thick scale leaves, 

 which in some cases are hairy, in others furnished with 

 resinous excretions, to serve the same purpose. No doubt 

 the thick cuticle of many leaves and twigs discharges a 

 similar function. 



Some plants secure a protection from excessive radiation 

 from the upper surfaces of the leaves during the night, by 

 folding them in various ways, so as always to expose as 

 little surface as possible, and that surface the one which is 

 least susceptible of injury by cooling. This so-called sleep 

 or nyctitropic movement plays a most important part in 

 the retention of heat, leaves that are prevented from carry- 

 ing it out perishing very rapidly. The features of this 

 behaviour will be examined more freely in a subsequent 

 chapter. 



Conduction of heat from the plant to its environment is 

 of constant occurrence, but it is exhibited most clearly 

 by plants that have an aquatic habit. The general inter- 

 changes that take place between a plant and the water in 

 which it lives range usually through only a few degrees of 

 temperature, and are so constantly going on that the 

 temperature of both tends to become readjusted after every 

 slight disturbance. In some cases, however, a very large 

 amount of heat is dissipated by these means, as we may 

 see in the fermentation of a saccharine solution by yeast. 

 The metabolic processes of the latter, incident upon its 

 nutrition and respiration, are so vigorous that a very large 

 amount of energy is liberated by and during the decom- 

 position of the sugar, and this takes very prominently the 



