40 THE RUBBER TREE BOOK 



As Tennyson said of the flower which he had picked out of 

 the crannies of the old stone wall : 



" I hold you here, root and all, in my hand, 

 Little flower but if I could understand 

 What you are, root and all, and all in all, 

 I should know what God and man is." 



The main function of the stem of a tree is to support the foliage 

 which it produces, and to serve as a means of communication 

 between the roots and the leaves. As has already been stated, 

 and as will be seen from the diagram given here, the stem of the 

 Hevea tree is composed of various layers: (i) The heart-wood; 

 (2) the sap-wood; (3) the cambium; (4) the inner layers of the 

 bark (cortex), in which lie the laticiferous vessels; and (5) the 

 outer layers of the bark (the bark proper) . 



Heart-wood. The heart-wood, which, as its name denotes, 

 forms the centre of the stem, is dead, and contains no living cells 

 or protoplasm. As the coral island is built up by successive 

 generations of living coral cells on the dead bodies of their pre- 

 decessors, so have these patriot cells of the heart-wood perished, 

 that over their dead bodies fresh troops of cells may proceed to 

 build the tree-city. Trees of nearly every species can continue 

 to flourish and show no signs of decay although the heart- 

 wood may have rotted away or be eaten out by white 

 ants. 



Sap-wood. The layers of sap-wood are composed of tubes 

 (vessels) and of living cells, and surround the heart-wood. In 

 trees of larger growth, like the Hevea, it forms a thick ring, 

 and helps to account, to a considerable extent, for the great 

 vitality of the Hevea trees. It is through the vessels in the sap- 

 wood that the water and nutrient salts derived from the roots 

 find their way upwards to the leaves and other parts of the 

 trees. 



Cambium. The cambium layer, which immediately over- 

 lies the sap-wood, is very thin, probably not more than the 

 thickness of an ordinary sheet of paper. Yet it forms one of the 

 most important sections of the tree. It is another exemplifica- 

 tion of the fact that dimension is often in inverse ratio to 

 importance. When bark is torn away from a tree, the cam- 

 bium, being very delicate, is ruptured, and appears as a slimy 



