TEMPERATURE AND ITS CONDITIONS = 383 
and imperfect. We do not find that an increased loss of 
heat stimulates metabolism in such a way as to set up 
destructive processes, which should liberate heat to com- 
pensate for the loss. On the contrary, such increased de- 
compositions are promoted by a rise instead of a fall of 
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 as a 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 
considerably 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 contaims. 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, 
