8 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



rings." The generally accepted explanation of this 

 is somewhat as follows. In the spring-time and early 

 summer, the cambium-cells begin to divide, and those 

 on the inner side of the cylinder of cambium gradually 

 become converted into tracheides (excepting at a few 

 points where the cells add to the medullary rays), and 

 this change occurs at a time when there is (i) very 

 little pressure exerted on the inner parts of the trunk 

 by the cortex and corky bark, and (2) only compara- 

 tively feeble supplies are derived from the activity of 

 the leaves and roots, in the still cool weather and 

 short days with little sunlight. In the late summer, 

 however, when the thickened mass of wood is com- 

 pressed by the now tightened jacket of elastic bark 

 which it has distended, and the long, hot bright sunny 

 days are causing the numerous leaves and roots to 

 supply abundance of nutriment to the growing cam- 

 bium cells, it is not surprising that these cells cannot 

 extend themselves so far in the radial direction (i.e. 

 in a line towards the centre of the compressed stem), 

 and that their walls are thickened by richer deposits 

 of woody material supplied quickly to them. 



As the winter approaches, the cambium ceases to 

 be active, and it then remains dormant for several 

 months. When its cells are awakened to renewed 

 growth and division in the following spring, they at 



