OF TEMPERATURE. 85 



the pollen is impaired or destroyed, and no produc- 

 tion of seed takes < place, 



111. Should the temperature be so much lowered 

 as to result in freezing, a destruction of some plants 

 and injury to others take place, owing to physical 

 causes quite different from those whose operation has 

 been explained in the last paragraph. In what de- 

 gree frost acts upon the vegetable fabric depends upon 

 the specific nature of a plant ; the least frost destroy- 

 ing some species, while others, under equal circum- 

 stances, endure any known amount of natural cold : 

 but, as general phenomena, it is in evidence, that, 

 when a plant is frozen, the following effects are pro- 

 duced : — 1st, The fluids contained within the cells of 

 tissue are congealed, and consequently expanded; 

 2nd, Such expansion produces, to some extent, a la- 

 ceration of the sides of the cells, and impairs excita- 

 bility by the unnatural extension to which the sides 

 of the cells, if not lacerated, are subjected; 3rd, It 

 expels air from the aeriferous cavities ; 4th, It also 

 introduces air, either expelled from the air passages, 

 or disengaged by the glacial decomposition of water, 

 into parts naturally intended to contain fluid ; 5th, 

 The green colouring matter and other secretions are 

 decomposed: 6th, The vital fluid, or latex, is de- 

 stroyed, and the action of its vessels paralysed ; 7th, 

 The interior of the tubes, in which fluid is conveyed, 

 is obstructed by a thickening of their sides. These 

 phenomena may be considered in part mechanical, in 

 part chemical, and in part vital. The two latter are 

 beyond control, and probably depend either upon the 



