INFLUENCE OF CLIMATE ON PLANTS. 
87 
attests, young trees are generally tender, and particularly pervious to frost : whereas, 
in proportion as they become older, evaporation is increased, — their tissue is 
consolidated and protected hy a thicker and tenser cuticle, — and they are thus 
rendered far more secure from any danger arising from a decrement of temperature. 
The tenderness in this instance proceeds from the same cause as that of the newly- 
formed wood previously noticed : mz.^ an undue propulsion of fluids into the young 
shoots at a period when they are only partially organized, and consequently, when 
the texture of their vesicles is not sufficiently rigid to resist the pressure of the 
accumulated sap, during the interruption of the transpiratory process. 
Young plants have generally a larger absorbent superficies on their roots, till 
these have become fully hardened, than they present relatively to the atmosphere 
for ensuring the necessary exhalations and concretion ; and their supply of fluids is 
thus much greater than they can dispose of, if at all checked by cold. The short 
distance at which their roots lie from the surface of the soil, may also be assigned 
as a reason for their peculiar susceptibility ; because, when their rootlets have 
penetrated to a considerable depth, they are enabled to imbibe fluids from the lower 
strata of a much higher temperature than those of the surface, and these, by their 
ascension, necessarily tend to preserve a due degree of warmth and vitality in the 
plant. By a parity of reasoning, those plants whose roots extend themselves 
horizontally, cannot possibly be so well prepared for sustaining cold, as others, the 
roots of which strike downwards in a perpendicular direction. 
As a brief account of the manner in which frost operates so inimically on 
vegetation, cannot prove otherwise than interesting, it may here be attempted. 
The substance of plants is mainly composed of cellular and vascular tissue, the 
individual vesicles and vessels of which are filled with fluids. By the congelation 
and consequent expansion of these fluids, the membrane which envelops and 
confines them is ruptured, and disorganization is thus effected. The subsequent 
action of heat upon the parts affected, causes an increase of evaporation commen- 
surate with its intensity ; and this, having then nothing to oppose or regulate its 
progress, continues till decomposition and volatilization are thoroughly completed. 
When the degree of frost is very trifling, and no derangement of the organic 
structure of the plant occurs, there are means of modifying or even counteracting its 
prejudicial influence. Every gardener is cognizant of the fact, that the external 
application of cold water to a plant newly frozen, will, in cases where the organiza- 
tion is uninjured, prevent fatal consequences. It is upon the same principle that 
snow, applied freely to a frozen member of the human body, restores life and 
animation. In both cases, the vital fluids are congealed, and a rapid thaw, such 
as the action of heat will induce, would have a similar effect to that of a more 
intense degree of frost, viz. expand too precipitately, and cause a disruption of the 
membraneous covering of the tissue. On the other hand, by immediately washing 
or sprinkling with cold water the part frozen, the thaw is rendered more gradual, 
and circulation is afterwards recommenced. 
