74 BRITISH PLANTS 
For every plant there is a certain limit of temperature 
below which it cannot live, and for some hibernating 
structures like dry seeds the temperature which they can. 
survive is very low indeed. Of all living things, the seed 
is able to resist the greatest cold, and for this reason it is 
looked upon as the most pronounced xerophytic structure 
in Nature (p. 59). Geophytic structures such as bulbs, 
rhizomes, etc., also survive refrigeration. Herbaceous 
organs, on the other hand, suffer severely at low tempera- 
tures, and in some cases, especially when they contain 
a great deal of water, they are killed off at the first 
frost. 
Heat and cold, as such, seem to evoke little or no 
protective adaptations in plants. They act indirectly 
through the water-factor, and cannot be disassociated from 
it. Cold, for example, diminishes absorption by the 
roots, and therefore necessitates a control of transpiration ; 
cold regions are therefore regions of physiological drought, 
and their vegetation is xerophytic. Heat, on the other 
hand, increases the activity of all the functions, absorp- 
tion and transpiration alike ; but as the area which ab- 
sorbs is generally very much less than the area of the 
surfaces which transpire, and as a rise in temperature 
increases all the factors favouring transpiration, a high 
temperature tends to make the loss by transpiration 
excessive and dangerous. Hence hot regions, unless they 
are constantly moist, are regions of physiological drought, 
and the vegetation is xerophytic. Thus, extreme heat 
and extreme cold both lead to xerophytic adaptation— 
the latter always, and the former when the water-supply 
becomes insufficient for the increased demands. Apart 
from this, however, it is pretty clear that some plants are 
constitutionally better able to stand cold than others ; 
those are most delicate whose tissues contain much 
water: plants accustomed to alpine surroundings must 
naturally be better able to stand cold than the inhabitants 
of warmer regions. There are some common plants, also, 
which flourish during winter without showing in their 
structure any particular marks of protective adaptation. 
The chickweed, for example, lives throughout the winter, 
survives frosts, and yet shows no special adaptations by 
which we can explain its immunity ; even its buds are not 
protected. It must therefore be constitutionally hardy. 
