334 VEGETABLE PHYSIOLOGY 



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 

 cells, they may continue to live, but otherwise the organ is 

 'killed. The cells become flaccid and the protoplasm at 

 once ceases to have the power of maintaining them in the 

 turgid condition. 



The effect of the absence of moisture in enabling vegetable 

 organisms to resist cold has recently been examined in the 

 case of seeds. Several kinds of these have been found to 

 be capable of germinating after immersion for several hours 

 in liquid hydrogen, the temperature of which is the lowest 

 at present known. 



A similar effect is found at the other end of the scale. 

 If seeds are heated very gradually some will withstand a 

 temperature of 98 C. The gradual loss of water is a 

 necessary condition for this immunity, for when the heat- 

 ing is conducted so quickly that the water is not driven off 

 at a low or moderate temperature, the treatment is fatal 

 in all cases. Under conditions of gradual heating, their 

 temperature being maintained at 60 C. for twenty-four 

 hours, seeds have been found capable of germinating after 

 a subsequent exposure to 98 C. lasting for ten hours. 



Spores of bacteria and of fungi have a great power of 

 resisting high temperatures, and this is probably also asso- 

 ciated with a considerable degree of dryness. They can 

 withstand boiling in water for some time, but it is probable 

 that the reason why they are not destroyed is that their 

 walls successfully resist the passage of water into their 

 interior. 



We are unable at present to explain in detail the causes 

 of the death of protoplasm under the conditions of extremes 

 of temperature. We can only say that under these con- 



