Effects of Frost on Plants. 27 



tenuity, but in .which the motion of the latex may be seen beau- 

 tifully with the one-eighth of an inch object-glass of an achro- 

 matic microscope. Upon being thawed after freezing, all this 

 apparatus is found reduced to some misshapen separate sacs of 

 fine grumous matter in which no motion can be detected. That 

 these vessels lose their vitality after freezing, may indeed be seen 

 without the aid of a microscope ; for if a stem of a Ficus elastica, 

 or an Euphorbia, or any such plant, which discharges an abun- 

 dance of milk when wounded, be first frozen and then thawed, 

 no milk will follow the incision. 



" From these facts, I think we must draw the conclusion, that 

 the fatal effect of frost upon plants is a more complicated action 

 than has been supposed ; of which the following are the more 

 important phenomena : 



" 1. A distension of the cellular succulent parts, often attended 

 by laceration, and always by a destruction of their irritability. 



"3. An expulsion of air from the aeriferous passages and cells. 



" 3. An introduction of air, either expelled from the air-passa- 

 ges or disengaged by the decomposition* of water, into parts in- 

 tended exclusively to contain fluid. 



" 4. A chemical decomposition of the tissue and its contents, 

 especially the chlorophyll. 



" 5. A destruction of the vitality of the latex, and a stoppage 

 of the action of its vessels. 



" 6. An obstruction of the interior of the tubes of pleurenchy- 

 ma (woody fibre) by the distension of their sides. 



" These phenomena may be considered in part mechanical, in 

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

 control. * * * The mechanical action of frost may, how- 

 ever, undoubtedly be guarded against to a great extent. It is 

 well known that the same plant growing in a dry climate, or in a 

 dry soil, or in a situation thoroughly drained from water during 

 winter, will resist much more cold than if cultivated in a damp 

 climate, or in wet soil, or in a place affected by water during 

 winter. Whatever tends to render tissue moist will increase its 

 power of conducting heat, and consequently augment the sus- 

 ceptibility of plants to the influence of frost ; and whatever tends 



^ Or rather disengaged from the water, which held it in solution, during the act 

 of freezing. A. G. 



