TEMPERATURE AND ITS CONDITIONS 327 
temperature. On the other hand again the processes of 
growth, repair, and constructive metabolism are also in- 
creased as the plant becomes warmer. Conversely, the 
setting up of metabolic activity raises temperature. A rise 
which can be measured by a delicate thermopyle follows 
the cutting or wounding of a potato, or the bulb of an 
onion. The metabolism set up is chiefly respiratory, for it 
is accompanied by an increased output of carbon dioxide. 
We can thus speak of what takes place asa tendency to 
economise and distribute heat, rather than as a process of 
regulation. Even the distribution of heat, whether on its 
first absorption or after subsequent fixation and liberation, 
is so unequal that different parts of a plant may differ con- 
siderably as to their temperature. 
As we have seen, life is possible within certain limits of 
temperature only. The maintenance of a healthy life 
depends upon the adequate discharge of various functions, 
each of which needs again a certain range. The limits 
within which life is possible do not necessarily coincide 
with those which are appropriate to every function. Out- 
side the latter, however, a plant becomes unhealthy and 
eventually perishes, falling a victim to the attacks of 
internal or external adverse influences. 
We do not find that all plants, or indeed all parts of 
plants, show the same amount of resistance to the extremes 
of heat and cold. The injury which any part of a plant 
experiences under such conditions, depends very much upon 
the amount of water which it contains. If more than a 
trace of the latter is present, the formation of ice which 
takes place below 0° C. may lead to rupture of the cells, the 
ice being usually deposited outside them. A considerable 
disturbance of the osmotic equilibrium of the sap may 
occur, setting up secondary injuries. The protoplasm 
becomes disorganised also at the low temperature. 
After the freezing of a tissue has taken place, a subse- 
quent rise of temperature leads to a process of thawing. 
This in many cases is more fatal to it than the freezing, but 
the effect depends largely on the rapidity of thethawing. If 
