RUBBER PLANTING 41 



leaves in a state of very weak solution. The perfected 

 food materials are carried down through definite chan- 

 nels, known as phloem tubes, in the inner part of the 

 bark, by a downward stream of sap which is entirely 

 independent of the upward stream. In species which 

 produce latex there is also present in the inner bark 

 a special system of minute vessels or tubes, which 

 contain an emulsion of rubber and other substances. 

 This system is closed, and is entirely separate from both 

 the above-mentioned sets of channels, having nothing 

 to do with either of them, although in position it is 

 closely associated with the phloem tubes which carry 

 the downward sap current. The upward and downward 

 streams of sap are found in all trees, but latex tubes 

 occur in only a comparatively small number of species. 



A most important organ of the tree is the cambium. 

 It is in this layer of very delicate cells that the growth 

 and formation of new tissues are continually going for- 

 ward. On the inner side of the cambium fresh layers, 

 consisting of newly constructed vessels, fibres and cells, 

 are constantly being added to the wood. On the outer 

 side of the cambium, and consequently on the inner side 

 of the bark, similar new additions are constantly being 

 made to the latter. These additions to the bark provide 

 for the increased strain on the capacity of the conduct- 

 ing vessels, consequent on the general growth of the 

 tree; and replace the losses occasioned by ordinary 

 wear and tear, or in the case of cultivated rubber trees 

 by the tapping knife. In addition to the new channels 



