290 DISEASES OF TREES 



months to the action of air and sun, as was the case, for instance, 

 in the winter of 1879-80. In this case drought alone was 

 accountable for death. Even in the course of the winter 1879-80 

 the leaves of middle-aged spruces and silver firs became brown 

 and died where the foliage was exposed to the direct rays 

 of the sun, and where constant air-currents encouraged trans- 

 piration, as, for instance, on the southern edges of woods, on 

 railway embankments, or on spruce hedges, &c. It was said 

 that in Alpine regions which were much exposed to the south 

 wind even old woods of silver firs succumbed entirely to the 

 influence of the frost. In my opinion these phenomena can 

 only be explained by the circumstance that repeated thawing and 

 accelerated transpiration are induced in the leaves by the direct 

 action of the sun during the bright wintry weather that usually 

 prevails in these parts, or by the warm south winds, as the 

 case may be, and that the leaves wither because they are unable 

 to obtain any water from the stems which have been frozen 

 under the influence of long-continued and severe cold. Many 

 of the phenomena accompanying the defoliation of pines, as well 

 as the death of the branches of old pines, may also be explained 

 in this way. The injurious effects of repeated thawing and 

 freezing, long-continued frost, or strong drying winds are to be 

 explained by the scarcity of water that results from the inter- 

 rupted or at least reduced passsage of water. 



The limits of forest growth in northern latitudes and in 

 mountainous regions are determined not so much by the low 

 temperature as by the action of drought on those parts of the 

 tree that project from the snow during the long period of 

 vegetative inactivity. On this account, too, we find that the 

 limits of tree-growth are reached at a considerably lower 

 elevation on south and west slopes, where the action of the sun, 

 augmented as it is by reflection from the snow, is stronger than 

 on north and east slopes. 



We are still awaiting a satisfactory explanation of the 

 familiar fact that trees, especially exotic conifers, are more easily 

 killed by frost in a wet situation than in a dry one, and that in 

 general the more succulent parts of plants are more liable to suc- 

 cumb to frost than those portions which are comparatively dry. 



When trees have suffered from frost during winter the 



