II.] ITS PROPERTIES AND VARIETIES. 31 



factors we cannot here speak, since they would carry 

 the argument too far for the space at command ; some 

 of them are obvious, but there are correlations of a 

 subtle and complex nature also. 



First as to temperature. The dormant condition of 

 the cambium in our European winter is directly 

 dependent on the low temperature : as the sun's rays 

 warm the environment, the cambium cells begin to 

 grow and divide again. The solar heat acts in two 

 ways : it warms the soil and air, and it warms the 

 plant. Wood, however, is a bad conductor of heat, 

 and the trunk of the tree is covered by the thick corky 

 bark, also an extremely bad conductor, and it would 

 probably need the greater part of the early summer to 

 raise the temperature of the cambium sufficiently for 

 activity in the lower parts of a tree by direct solar 

 heat : the small twigs, on the contrary, which are 

 covered by only a thin layer of cortex, and epidermis, 

 are no doubt thus warmed fairly rapidly, and their 

 early awakening is to be referred to this cause. The 

 cambium in the trunk, however, is not raised to the 

 requisite temperature until the water passing up 

 through the wood from the roots is sufficiently warm 

 to transmit some of the heat brought with it from the 

 soil to the cells of the cambium. This also is a 

 somewhat slow process, for it takes some time for the 



