II.] ITS PROPERTIES AND VARIETIES. 33 



plant, growing naturally at elevations where the 

 temperature of the soil is not high enough to 

 communicate the necessary stimulus to the cambium 

 until the end of May or June. Larches growing in 

 the lowlands, however, are apt to begin their renewed 

 growth in April, and frosted stems are a common 

 result, a point which (as the botanist just referred 

 to also showed) has an important bearing on that 

 vexed question the " larch-disease." 



The supply of oxygen to the cambium is chiefly 

 dependent on the supply of water from the roots, and 

 the aeration of the stem generally. The water begins 

 to ascend only when the soil is warm enough to 

 enable the root-hairs to act, and new ones to be 

 developed, and the supply of mineral salts goes hand 

 in hand with that of water. 



Now comes in the question of the sources of the 

 organic substances. There is no doubt that the 

 cambium at first takes its supply of food-materials 

 from the stores which have been laid by, in the 

 medullary rays and wood-parenchyma, &c., at the 

 conclusion of the preceding year ; and it is known 

 that special arrangements exist in the wood and 

 cortex to provide for this when the water and oxygen 

 arrive at the seat of activity. 



Assuming that all the conditions referred to are 



D 



