64 HARDINESS IN PLANTS. 
account of what is known about it may be found in Sachs’ ‘‘ Text Book 
of Botany,” p. 653. It appears that the capacity to resist frost may be 
due to several causes, either singly or in combination. Among these — 
are:—1. The proportion of water to solid matter in the cellular tissue. © 
2. The chemical constitution of the solid matter and the cell-sap, which 
varies slightly in different species. 3. The proportion of foliage to roots, 
and of stomata toleaf-surface. 4. The more or less dense structure of the 
epidermal tissue. Plants are killed by frost not so much through the 
rupture of their tissues in consequence of the expansion of the water 
into ice, as was formerly supposed, but rather through the structural 
and chemical changes which take place in consequence of the normal 
proportion of water being removed from the cell-walls, the cell-sap, and 
the protoplasm. Death is due quite as often to the thawing as to the 
freezing. A rapid thaw will kill many plants which would have 
survived a slow one, because the water which has been separated from 
the other constituents of the vegetable substance may be re-absorbed 
if the thawing is sufficiently gradual, but cannot be re-absorbed if it is 
too sudden. 
Take a cup-full of starch-paste. Freeze it into a solid mass, and 
then thaw it quickly. It will not return to the condition of paste, but 
will have become a soft sponge, with water in the interstices. So the 
cellular tissue of the plant, when its normal proportion of water has been 
once frozen out cannot always re-absorb it, or return to its original 
condition. The cells shrink, become disorganised, and unable to carry on 
the functions of life. 
Tt is well known that some chemical solutions will freeze much more 
easily than others. River water is coated with ice sooner than sea water. 
In like manner the sap of some plants freezes, no doubt, at a lower 
temperature than that of others. Itis almost impossible to freeze a moss. 
The solid cell-walls and protoplasmic bodies contain “water of 
imbibition ” as crystals contain ‘‘ water of crystallisation.” This water is 
held between the molecules of the solid by the force of cohesion. If the 
proportion of water is large as in succulent plants and young foliage, the 
force of cohesion is weak, and the water is easily separated. Hence such 
plants and foliage are less hardy than those of adenser and dryer nature. 
Again, some plants ‘“ transpire” or exhale watery vapour more rapidly 
than others. This seems to be a frequent character of plants indigenous 
to warm climates. To maintain the supply of moisture from the roots 
the soil must be damp and warm. In a climate where the soil becomes 
cold or frozen such plants would die from thirst. 
Plants may become ‘“ acclimatised” by a gradual modification of 
some of their original functions, so as to adapt themselves to the average 
temperature of a new climate. The Portugal cabbage, (Cowve Tronchuda,) 
which some years ago was a comparatively tender variety, is now much 
hardier. 
