34 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



favourable, the cambium cells become filled with 

 water in which the necessary substances are dissolved, 

 and distended (become turgid, or turgescent, as it is 

 technically called) sufficient for growth. Speaking 

 generally, and with reference chiefly to the trunk of 

 the tree, which yields the timber, the distension of the 

 cells is followed by growth in the direction of a radius 

 of the stem, and division follows in the vertical plane, 

 tangential to the stem. Then the processes already 

 described in connection with Fig. 5 repeat themselves, 

 and the trunk of the tree grows in thickness. 



Now it is obvious that the thickening of the mass of 

 timber inside the cylinder of cambium must exert 

 pressure on the cortex and bark must distend them 

 elastically, in fact and some ingenious experiments 

 have been made by De Vries and others to show that 

 this pressure has an effect in modifying the radial 

 diameter of the cells and vessels formed by the 

 cambium. Several observers have promulgated or 

 accepted the view that the differences between 

 so-called spring and autumn wood are due to the 

 variations in pressure of the cortex on the cambium, 

 but the view has lately gained ground, based on 

 experimental evidence, that these differences are 

 matters of nutrition, and a recent investigator has 

 declared that the thick-walled elements and small 



