Gums, Resins, 



390 



[May, 1910. 



In Hevea, on the other hand, the tubes 

 arise from rows of cells through the 

 breaking down of the intervening walls. 

 The perforations are not always com- 

 pletely formed, so this, the articulate 

 system, is relatively disconnected com- 

 pared with the other. 



A wound in a tree containing the first 

 arrangement will therefore most likely 

 drain a larger area of laticiferous tissues 

 than one in a tree of the second type. 

 This doubtless accounts for the greater 

 flow of latex from an initial incision in 

 the trunk of Castilloa compared with 

 that from one in Hevea ; but it is difficult 

 to explain the wound-response in the one 

 and not in the other. Perhaps Hevea has 

 % much more extensile, though less com- 

 municative, system than Castilloa ; or in 

 other words, a trunk of Hevea has a 

 much larger number of tubes, and so 

 holds a greater quantity of latex than 

 a corresponding one of Castilloa. At the 

 first tanpiug the latter gives up practi- 

 cally all its latex, on account of the 

 tubes freely communicating with one 

 another; whilst the former only yields 

 up a very small portion of its total 

 quantity of latex, through the com- 

 paratively disconnected nature of its 

 system. Thus from a single trial 

 Castilloa appears the better yielder. On 

 retapping in a few days ' time no more 

 latex exudes. The tubes apparently do 

 not refill with liquid, and so probably 

 collapse. In Hevea, however, a fresh 

 set of tubes will be served at the second 

 tapping, and if the new incision be made 

 near the old one, the ducts here will 

 probably be surcharged with latex 

 owing to a great infiltration of fluid 

 caused by the previous wounding ; thus 

 from such an incision an increased 

 quantity of latex will flow. 



A detailed microscopic study of the 

 laticiferous systems of these two trees 

 might shed some light on the above 

 suppositions. Manihot, however, has a 

 system similar to that of Hevea, and yet, 

 as far as it has been investigated, it 

 shows no wound-response. Johnson* 

 experimenting with this tree in Portu- 

 guese East Africa, has failed to get it to 

 respond to multiple tapping. 



The Function of Latex.— A few words 

 on the question of the function of latex 

 are called for here. It is still largely a 

 problem awaiting solution. 



A nutritive function for the latici- 

 ferous tubes was at one time upheld. 

 They were supposed to act as conductors 

 of plastic material, especially of pro- 

 teins, and were considered in some cases 



* W. H. Johnson, India Rubber Journal, 1008, 

 Vol. xxxv., p. '209. 



partly to replace the sieve tubes. Ad" 

 herence to such a view has lost ground 

 in recent years. 



Spence,* however, has recently revived 

 the nutritive view on somewhat start- 

 ling lines. His studies on the oxidising 

 enzymes of latex has led him to regard 

 the caoutchouc as a food reserve, which 

 by means of these ferments may be 

 oxidised and broken down into simple 

 carbohydrates for the plant's use. 

 Physiologists will require much evidence 

 before accepting such a novel theory. 



That the tubes conduct or store food 

 materials for the plant seems doubtful- 

 Primarily the latex may be regarded 

 rather as a waste product, and the tubes 

 containing it as genetically related to, 

 and a further development of, secretory 

 sacs. But the substitution of an exten- 

 sive system of communicating tubes in 

 place of isolated sacs apparently implies 

 the adoption of some new function in 

 addition to that of removiug the waste 

 products of metabolism. A conducting 

 function is the one which suggests itself. 

 The tubes may form channels for the 

 conveyance and storage of water. Lati- 

 ciferous plants, at any rate the arbores- 

 cent ones, are distinctly numerous in 

 the tropics, where transpiration at times 

 is excessive, especially during the dry 

 season. 



Again, the theory has been advanced 

 that the latex serves as a protection 

 against insects and fungi. In respect to 

 an insect, a puncture or bite will result 

 in an outflow of latex, which may inter- 

 fere with its further operations or prove 

 distasteful to it. The penetration of a 

 fungus through a wound may be pre- 

 vented by the latex, which oozes out, 

 forming an impenetrable layer. This 

 supposed protective function for latex 

 must be investigated separately for each 

 species in its original surroundings. The 

 laticiferous system may have been 

 evolved to repel certain foes occurring 

 in the natural habitat of the plant, and 

 yet be ineffectual against other enemies 

 which the species may meet in a new 

 environment. 



The theory of water-storage and con- 

 duction is perhaps the most plausible. 

 The watery nature of the latex in the 

 trunk of Hevea has been noticed to be 

 affected by the state of the soil- When 

 dry, the latex is thicker and flows out 

 less readily, suggesting that the tree is 

 drawing upon the reserve of water 

 accumulated in the laticiferous tubes. 

 In the alluvial regions of the Malay 

 States the tree yields latex very abund- 



* D. Spence, Bio-Chemical Journal, 1908, 

 iii., 179-81. 



