i.] GENERAL CHARACTERS AND STRUCTURE. 13 



towards the centre of the stem). After a time the 

 oldest of these cells (i.e. those nearest the centre of 

 the stem) cease to divide, and undergo changes of 

 another kind : the process of division is still going on 

 in the younger ones, however ; and so the radial rows 

 are being extended by additions of cells at their outer 

 ends. Of course, this is normally proceeding along 

 the whole area of the cylindrical sheet of cambium, 

 and therefore over the whole surface of the wood of 

 the stem and roots, with their branches. 



Confining our attention for the present to one of the 

 innermost, oldest cells of the cambium, which has 

 ceased dividing (aa in Fig. 5), we find that it enlarges 

 somewhat in the radial direction, and then its hitherto 

 very thin walls become thicker ; in fact, the protoplasm 

 in its interior absorbs food-materials, and changes 

 them into a peculiar substance which it plasters or 

 builds on to the inner sides of the cell-wall, so to speak, 

 until the wall is rendered much thicker. This thicken- 

 ing process is withheld at certain places only 

 the thin depressions or "pits" already referred to. 

 Two chief changes result now: (i) the whole of the 

 living contents of the young wood-cell gradually 

 become used up, and eventually disappear without 

 leaving any trace, their place being occupied by water 

 and air in most cases ; and (2) the thickening substance 



