TEMPERATURE AND ITS CONDITIONS 3881 
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 
