328 VEGETABLE PHYSIOLOGY 



destructive processes, which should liberate heat to com 

 pensate for the loss. On the contrary such increased 

 decompositions 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. We can rather speak 

 of what takes place as a tendency to economise and distri- 

 bute heat, 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 contains. If more than a 

 trace of the latter is present, the formation of ice which 

 takes place below 0C. 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 the thawing. If 

 it is so gradual that the water can be re-absorbed into the 



