TEMPERATURE. 189 



of the substances in the cells, accompanied by an eTolution 

 of carbon dioxide. 



247.— Death Caused by Low Temperature. In many 

 respects the results of too great a reduction of temperature 

 are similar to those produced by too great an elevation. 

 There is observed the same coagulation of the albuminoids, 

 resulting in the destruction of the power of the protoplasm 

 to imbibe water, and, as a consequence, in the loss of the tur- 

 gidity of the cells. Moreover, as in the case of injury from 

 high temperature, those cells which are the most watery are 

 the ones which, other things being equal, are injured most 

 quickly by a reduction of temperature. Embryo plants in 

 seeds, when dry, are able to endure almost any degree of low 

 temperature ; but after they have germinated, and the cells 

 have become watery, they are generally killed by a reduction 

 to, or a few degrees below, 0° Cent. (33° Fahr.). So, too, 

 the comparatively dry tissues of the winter buds and ripened 

 stems of the native trees and shrubs in cold countries are 

 rarely injured even ia the severest winters, while the young 

 leaves and shoots in the spring are often killed by slight 

 frosts. 



Death from low temperature is always accompanied by the 

 formation of ice-crystals in the succulent tissues ; these are 

 formed from the water of the plant, which is abstracted from 

 it in the process of congelation. Much of the water thus 

 frozen is that which fills the cavities (vacuoles) of the cells, 

 while some of it is that which moistens the protoplasm and 

 cell- walls. Now it is evident that the water in the large 

 vacuoles is much more easily congealed than that in the pro- 

 toplasm and cell-walls ; for in the latter the force of adhesion 

 between the molecules of protoplasm or cellulose and the 

 imbibed water offers a considerable resistance to the separa- 

 tion of the water in ice-crystals, and this resistance is greater 

 as the contained water is less. As the liquid in the vacuoles 

 is not pure water, but a mixture of several solutions, it freezes 

 at a lower temperature than water, and then, according to a 

 well-known law of physics, separates into pure ice-crystals 

 and a denser unfrozen solution. By a greater reduction of 

 temperature more ice-crystals may be separated out, and the 



