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leaf-blade and especially near the midrib being 

 often uninjured and green, while the rest of the 

 leaf is shrivelled and brown, i.e. just those portions 

 escape injury which can first intercept the water- 

 supply on its entrance into the leaf and which only 

 allow that quantity of water to pass on to the 

 more distant cells which is in excess of their own 

 needs. See Plate XX. The damage can often be well 

 seen in a nursery after a frosty night, no injury to 

 the leaves being usually noticeable until the plants 

 have been exposed to the bright sunlight for some 

 hours. It is obviously important that trees grow- 

 ing at high altitudes, where the leaves are often 

 exposed to the bright sunlight when the roots are 

 in soil still frozen, should be able to prevent trans- 

 piration as far as possible. We thus find that the 

 high-level Rhododendron campamdatum and Quercus 

 semecarpifolia have the undersurface of their leaves 

 covered with dense tomentum. With regard to 

 this form of injury it must be pointed out that 

 roots situated in the superficial layers of soil are 

 more subject to the effects of sudden changes in 

 temperature, while those in the deeper layers of soil 

 are chiefly affected by prolonged cold temperature. 

 Thus in some cases we find seedlings suffer more 

 than older plants, while in other cases the reverse 

 happens. In the latter case the superficial roots, 

 under the influence of a few hours hot sunshine, 

 having been sufficiently warmed to become active, 

 whereas the deeper roots are still in frozen soil. 

 An artificial covering- of straw, dead leaves, etc., 

 on the soil over the roots of a plant may often 

 prevent injury by frost, by moderating the tem- 

 perature of the surface soil, and it is probable 

 that the beneficial action of watering plants is 

 often due to the temperature of the roots being 

 raised sufficiently to become active thereby. 



(3) The splitting of stems of trees. When the temper- 

 ature is sufficiently low ice is always formed 

 in plants, usually in the intercellular spaces, but 

 in wood, where intercellular spaces are usually 

 absent, in the lumina of the wood elements. 

 As these spaces contain considerable quantities of 

 air besides water, the water finds ample space for 



