379 



I hope we see the direct connection of isoprene with Caoutchouc, 

 the latter being only the product of a series of polymerisations of 

 the former , the only link in the evidence that is wanting is to 

 understand how C0 2 goes into isoprene C 5 H 8 , but in extenuation 

 it may be urged that it is as difficult to show how C0 2 goes to 

 H. C. H. O. that is formic aldehyde. Granted this change takes 

 place primary to or secondary (which is of course more likely) to 

 the formation of starch in the leaf, we then must consider the mode 

 of flow of what now we must look upon as elaborated food ma- 

 terial. 



We know that the latex tubes of Hevea are placed in the u bark" 

 outside the cambium. So is it with the true vessels ; so that we 

 may look upon the laticiferous system as a modified system of 

 "vessels" in the bark. 



It was pointed out in the lecture that even "terrible mutilation " 

 of the bark did not seem to affect the condition of the tree. How 

 is this possible ? There may be some vessels intact in a tree of the 

 above description, but by far the larger majority are sure to be 

 destroyed, so that we must expect the tree to suffer. It has been 

 shown that a ring cut round a tree to the cambium materially alters 

 its condition. In view of these facts we are irresistibly driven to 

 the conclusion that the plant must be having a secondary or reserve 

 process of nutrition. Suppose that the reserve material is the latex 

 in its earlier stages. If now the bark be removed the latex by 

 reverse chemical changes may supply food to the tree, and if tapped 

 while the bark is badly • mutilated and so incapable of properly 

 nourishing the tree we would naturally expect a decrease of the 

 amount of latex obtainable, or a decrease in its rubber element, 

 though the quantity may be but little influenced, and this seems to 

 be so. 



I would not maintain, though I have suggested it, that the milk is 

 formed direct in the leaf ; it would be more correct to presume that 

 the formic aldehyde instead of polymerising to starch takes a dif- 

 ferent course and gets finally reduced to Caoutchouc. Let us see 

 what other circumstances are there that may be explainable in the 

 above light. What would be the effect (i) of continuous tapping? 

 The rubber element will be decreased owing to the fact that the 

 formed compounds require time to polymerise, perhaps Caoutchouc 

 is a product formed of the polymerisation of the other substances 

 while stagnant in the laticiferous system itself. If on the other 

 hand (2) the tapping is delayed, the Caoutchouc may have poly- 

 merised into a more stable substance and we know chemically that 

 pure Caoutchouc, when obtained by dissolving the crude substance 

 in chloroform and precipitating with alcohol", absorbs oxygen when 

 exposed. 



The general conclusion that can be drawn from the above observa- 

 tion is that undue mutilation of the bark may be fatal to the quantity 

 of rubber in latex, and that as long as the tree is in healthy condi- 

 tion in a proper soil the latex may be entirely withdrawn without 

 vitally affecting the physiological process of the plant. The whole 



