THE STEM 43 



layer on the surface of the wood. This cambium-layer is full 

 of intense life and activity. It is the layer to which the in- 

 crease in thickness of the stem, branches and roots is almost 

 entirely due. On its inner side it is continually adding cells to 

 the sap-wood, some of which join together to form conducting- 

 tubes, after losing their protoplasm, while on its outward side 

 it is always producing additional cortical cells, some of which 

 form laticiferous (latex-bearing) vessels, others sieve-tubes, etc. 

 It is easily wounded by careless tapping, and this, as stated 

 elsewhere, is resented by the cambium, and is one cause of the 

 formation of swellings and abnormal growths. 



Latex-bearing Layers. These are found, along with the 

 bast, in the inner bark (cortex) of the tree near to the cambium. 

 Alongside the laticiferous vessels are found the " sieve-tubes," 

 through which are conveyed downwards the food manufactured 

 by the leaves. Accompanying these are cells that probably 

 serve as temporary storehouses of reserve food for the use of the 

 tree. What the uses of latex are to the plant has long been a 

 matter of dispute. Some hold that the latex is excretory 

 matter, of no further use to the tree, while others stoutly object 

 to this view. It seems reasonable to suppose that the latex is 

 incidentally of some service to the tree, whether for temporarily 

 protecting wounds or as a protection against the attacks of 

 animals, insects or disease. 



Outer Bark (or Bark Proper). The coarse, dark-coloured, 

 dry bark on the outside of the tree acts as a protective shield 

 for all living cells within the inner bark, cambium and wood 

 of the tree. It resembles the outer walls of a fortified city. 

 The bark is being constantly attacked by fungi and other 

 enemies of the tree, and has to bear the burden of the defence. 

 Just as the engineers of a beleaguered city would do when their 

 outer defences were breached by an attack, so the tree sets itself 

 actively to reconstruct its walls, adding fresh bark where any 

 has been injured or cut away. 



