32 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



sun's rays to raise the temperature of the soil while 

 the days are short and the nights cold. It has been 

 shown that the cambium in the lower part of the 

 trunk of a tree may be still dormant three weeks or a 

 month after it has begun to act in the twigs and 

 small branches ; and it has also been pointed out 

 that trees standing in open sunny situations begin to 

 renew their growth earlier than trees of the same 

 species growing in shady or crowded plantations, 

 where the moss and leaf-mould, &c., prevent the sun 

 from warming the soil and roots so quickly. These 

 observations have also a direct bearing on the later 

 renewal of cambial activity in trees growing on 

 mountains or in high latitudes. Moreover, though I 

 cannot here open up this interesting subject in detail, 

 these facts have their connection with the dying off of 

 temperate trees in the tropics, as well as with the 

 killing of trees by frost in climates like our own. One 

 important practical point in this connection may be 

 adverted to. Growers of conifers are well aware that 

 certain species cannot be safely grown in this country 

 (or only in favoured spots) because the sun's rays 

 rouse them to activity at a time when spring frosts 

 are still common at night, and their young tissues are 

 destroyed by the frosts. Prof. R. Hartig has pointed 

 out a very instructive case. The larch is an Alpine 



