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 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 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 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 injurfes. 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 the thawing. If 



