14 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



built on to the inside of the walls undergoes changes 

 which convert it into true wood-substance in botanical 

 language, the walls become lignified. The cells b and 

 c in Fig. 5 illustrate what is meant. 



During all these changes, which occupy several or 

 even many hours or days, according to circumstances, 

 it will be observed that the definitive shape of the cell 

 is gradually completed, and then alters very little : 

 the prismatic cambium-cell has become a prismatic 

 tracheide, with thicker, lignified walls, and containing 

 air and water (with minute quantities of mineral 

 substances dissolved in it) in place of protoplasm and 

 nutritive substances. It is not necessary here to speak 

 of other and more subtle changes which may eventually 

 cause slight displacements, &c., of these cells. 



If I have succeeded in making the chief points in 

 this somewhat complicated process clear, there will be 

 little difficulty in explaining what occurs in other 

 parts of the cambium-cylinder. The cambium-cells 

 which happen to stand in the same radial row as the 

 cells of a medullary ray, simply go on being converted 

 into cells of the medullary ray, instead of into 

 tracheides ; cells which differ from the tracheides 

 chiefly in retaining their living contents and nutritive 

 materials i.e. substances like starch, proteids, sugars, 

 &c., which are used as food by the plant. Again, 



