The fact that the latex vessels are entirely separate from the channels in which the food-bearing sap is transported, gives rise to the natural question: what is the use of these latex vessels to the tree, and what is the precise function of the milky emulsion which they contain? This is a point upon which we are still very much in the dark. From the fact that the great majority of trees of all kinds get on perfectly well without latex, we are driven to the conclusion that this substance is not essential for the life of the plant. It is certainly the case that large quantities of latex can be removed without causing any visible injury to the health of the tree. The fact that the removal of latex stimulates the tree to the production of large additional quantities of this substance, having nearly the same composition as the latex originally present, suggests that the formation of latex cannot be regarded merely as the excretion of a waste product. On the contrary, the conclusion seems inevitable that latex is formed at the expense of valuable food material, containing as it does large quantities of carbon as well as a considerable percentage of proteid material. There seems to be no reason for doubting that the removal of latex causes a sensible drain upon the supply of food available for general growth.
It has been suggested that latex vessels serve as additional channels for the transport of food; and it has been stated that the phloem is poorly developed in laticiferous plants. This is not the case in the principal kinds of rubber trees grown upon plantations, all of Avhich possess a highly developed system of sieve tubes. Other functions which have been attributed to the latex vessels are storage of food, storage of water, storage of excretory products, or a combination of any two or more of the functions already named; or finally protection against the attacks of insects and other enemies. None of these suggestions, except the last, appear to be based on any satisfactory evidence. On the other hand, latex undoubtedly serves a protective function. Any area of bark which has been entirely depleted of latex, owing to disease or other causes, is usually soon riddled by boring insects. These seldom or never attack bark in which a good supply of latex is present Whether this function can be regarded as a sufficient cause to account for the separate evolution of such an elaborate system in several different branches of the vegetable kingdom, is a question which we may leave for the discussion of theorists.