INJURIES DUE TO ATMOSPHERIC INFLUENCES 289 



individual differences, and it is this fact that makes it possible 

 for us to acclimatize plants. As the capacity to resist frost 

 varies amongst individuals of the same species, just like any 

 other physiological or morphological peculiarity, it becomes 

 possible to acclimatize a tender plant by propagating hardy 

 varieties. It is also probable that hardier varieties are produced 

 in the struggle for existence that takes place along the line 

 which limits the natural geographical distribution of a plant, 

 where the increasing severity of the climate bars the way to 

 a further advance. From this it follows that in attempting to 

 introduce a certain species it must be advantageous to procure 

 the seeds from such frontier regions. 



Indigenous shrubs and forest trees suffer from winter frost 

 only under very exceptional circumstances. The roots of young 

 trees, more especially oaks up to four years old, may be killed if 

 severe and long-continued frost finds the lighter classes of soil 

 unprotected by snow or any other covering. The periderm on 

 roots is thinner than that on stems, and consequently the former 

 are less protected and moreover growth is active for a longer 

 period in roots, where it frequently continues till the middle of 

 winter, so that when frost occurs the tissues are not in the inert 

 condition which assists them to resist cold. Such plants burst 

 their buds in spring, but wither up whenever transpiration 

 from the delicate young shoots has exhausted the stock of 

 water. 



Shoots that have not completed their growth, especially the 

 Lammas shoots of the oak, suffer from winter frost. This is a 

 matter, however, that belongs to the second division of our 

 subject, which treats of the phenomena induced by frost in 

 plants that are affected while in a state of vegetative activity. 



Even our indigenous trees, more especially evergreen di- 

 cotyledons and conifers, may succumb during winter owing to 

 their supplies of water being abstracted not by cold but by 

 transpiration. 1 The absorption of water by the roots ceases 

 when the ground is frozen to a depth that is reached by the 

 roots of young plants. No harm is done if the trees are 

 protected above-ground against evaporation, by snow or any 

 other covering. They die, however, if they are exposed for 

 1 R. Hartig, UntersucJutngen, I. p. 133. 



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