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 110 

 protective adaptations in plants. The} 7 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. 



