EFFECTS OF COLD. 113 



natural tlia'wiiigmust of neoesaity be very gradual. In tKe summer of 

 1849, an evergreen Few HoUand Beech, well known to be tender, was 

 planted experimentally on the north of a ruined wall. In th.e 

 succeeding winter it was remarked that the upper part of it was not 

 screened from the sun ; but about three-fourths of it next the ground 

 was perfectly screened. On the approach of spring it was found that the 

 part exposed to the sun was kiRed to the level of the wall ; while the 

 parts below the level were unchanged even in colour. When we add 

 this to the cases of Fuchsias, Camellias, Tree Pseonies, &c., now becoming 

 so generally well known, it seems impossible to doubt that, the vitality of 

 plants and animals being the same, the same methods of treatment 

 which are known to be requisite in the one case are equally effectual in 

 the other. 



The congelation of the aqueous particles contained in plants 

 is in itself sufficient to cause such a derangement of function 

 as may end in death, and other supposed causes may he left 

 out of consideration. It will thus follow that, omitting 

 differences arising out of the peculiar nature of different species, 

 plants mil suffer from frost in proportion to the abundance and 

 fluidity of their secretions ; those whose tissue is driest, and 

 whose secretions are most dense, being the most capable of 

 resisting frost. Hence young shoots are destroyed by a degree 

 of cold which does not affect old shoots of the same species ; 

 and hence, also, the diminished capability of "unripe" shoots, 

 or of plants growing in wet situations, or of trees when they 

 first begin to vegetate, of enduring extreme cold.* 



The effect of cold is, as has been seen, to diminish excita- 



* M. De Candolle gives the following as the laws of temperature with respect to its 

 influence upon vegetation : — 



1. All other things being equal, the power of each plant, and of each part of a plant, 

 to resist extremes of temperatnre, is in the inverse ratio of the quantity of water they 

 contain. 



2. The power of plants to resist extremes of temperature is directly in proportion 

 to the viscidity of their fluids. 



3. The power of plants to resist cold is in the inverse ratio of the rapidity with 

 wliich their fluids circulate. 



4. The liability to freeze, of the fluids contained in plants, is greater in proportion 

 to the size of the cells. 



6. The power of plants to resist extremes of temperature is in a direct proportion 

 to the quantity of confined air which the structure of their organs gives them the 

 means of retaining in the more deUcate parts. 



6. The power of plants to resist extremes of temperature is in direct proportion to 

 the capabilhy which the roots possess of absorbing sap less exposed to the external 

 influence of the atmosphere and the sun. 



