OF TEMPEEATUEE. 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 la+.ter are 

 beyond control, and probably depend either upon the 



