The Origin and Occurrence of Rubber. 189 



The most obvious suggestion relates to the conservation of water, 

 and it seems quite possible that the rubber may act as a sort of blanket, 

 reducing to some extent the passage of water to the outer zones of tissue 

 and consequently to the outside of the plant, and as a storage material. 

 The slower deposition of rubber in irrigated plants and its behavior in 

 Castilloa elastica under similar circumstances lend a modicum of support 

 to this view. Rubber, as is well known, will take up and retain a certain 

 amount of water with considerable tenacity. One would be encouraged 

 to hold this view if rapidly grown field seedlings with much less than the 

 normal amount of rubber had not been known to pass successfully through 

 a long period of drought, indeed much longer than usual. Further, mari- 

 ola appears to be as well equipped for resisting drought as guayule, but 

 contains a very small amount of rubber. The obvious objection that the 

 mariola has some other means to the end would in this case, I believe, be 

 difficult to demonstrate, and as difficult to refute. We are here in the 

 field of teleological speculation. 



Spence (1908), studying latex, found that this contains oxidases 

 capable of acting upon caoutchouc, and argued that this material may 

 therefore serve as a reserve food material. 1 Similar enzymes probably 

 occur in the guayule, but it is safe to remark that in this plant, once the 

 rubber is laid down, it is there to stay, as shown by its abjection in com- 

 pany with the bark-tissues. Even in the cells adjacent to the active cam- 

 bium, or other physiologically active tissues, the amount is never reduced, 

 while, if of use as a source of energy to the growing twigs, we should find 

 some evidence, analogous to that seen in the starch-content of growing 

 twigs, that there is translocation. But such evidence is quite lacking. 

 Whatever may prove to be true of latex plants, therefore, there does not 

 appear to be the slightest evidence that rubber is in any sense a food 

 material in the guayule. 



This view has recently (1909) been again brought into question by 

 Spence: 



The fact that the caoutchouc, or rubber, does not occur in any definite latex 

 system in the guayule, but in the parenchymatic cells of the medullary rays and 

 cortex, and further, that the amount of rubber from the dried plant varies con- 

 siderably from one period of the year to the other * * *, seems at once to 

 suggest to my mind that the rubber must have an important function in meta- 

 bolic processes. That the rubber is cast off partially and in a modified form in the 

 bark, as Professor Lloyd has pointed out, does not in any way weaken the evidence 

 of my theory, and from experiments which I have recently made I have found 

 that young Ficus elastica trees, grown in an atmosphere and soil free from carbon 

 dioxide, gradually drew upon their milk, which became nothing more than water 

 after a few weeks' time. 2 In any case * * * the guayule plant shows very 

 clearly that we can hardly retain the theory that the latex merely affords protec- 

 tion to the plant against internal injury and moisture in time of drought; in 

 guayule there is no secretion on injuring the plant, and no reserve water-supply, 

 though the rubber is there all the time. * * * " 3 



1 See also Cook, 1903. 



2 There has been a long controversy on the function of latex, for an account 

 of which see Tschirch, 1906. 



3 The quotation was printed in the past tense and third person. I have 

 made it into the first person, present. The italics are Spence's. (Lloyd, 1909. Dis- 

 cussion, p. 14 t). 



