6*2 VITAL FUNCTIONS OP PLANTS. [LECT. IT. 



mals. It is sufficient, however, to protect them 

 from the effects of frosts, that would otherwise 

 freeze the sap contained in the vessels of many of 

 them, and destroy their organization : for, we find 

 that plants live during frosts which freeze the 

 deepest lakes ; and dry up the moisture of every 

 substance not endowed with vitality. It is never- 

 theless true, that some plants may be even frozen 

 during winter, and yet be so tenacious of life as 

 to revive in the spring; but this extraordi- 

 nary circumstance is not peculiar to vegetable 

 beings, as a caterpillar, for instance, may be frozen 

 and yet live, after being thawed. The tender 

 shoots that contain much moisture, and sickly 

 branches, are however destroyed by severe frost : 

 for the vitality in the latter case is not suffi- 

 cient to preserve the innate heat ; and in the 

 former its power is not great enough to prevent the 

 freezing of the superabundant moisture contained 

 in the substance of the shoot ; and as all fluids 

 expand in the act of freezing, this effect tears 

 asunder and destroys the delicate vegetable vessels. 

 Nor can tender and sickly animals resist severe 

 frost more than vegetables, but are immediately 

 frost-bitten. When this happens, the part morti- 

 fies; and, although the vigour of the general habit 

 be supported, yet, the frost-bitten part can never 

 again be restored to its former state, but is sepa- 

 rated as a dead slough from the neighbouring 

 sound parts. The san.e effect is produced in a 



