The Origin and Occurrence of Rubber. 177 



the contents of the cells ruptured during the sweep of the knife agglomer- 

 ate and stick in irregular masses to the section. It is also to be suspected 

 that particles of rubber displaced from one cell may remain attached to 

 other cells in such a manner as to simulate an original position in them. 

 This danger is greater where the particles are small, since with smaller 

 size the chance against agglomeration is greater. With some experience, 

 however, this general difficulty is reduced, so that, with proper observa- 

 tion, mistakes are easily avoided. The guiding principle of observation is 

 simply to confine study to uninjured cells. 



GENERAL DISTRIBUTION OF RUBBER IN THE PLANT. 



It has been known for some years l that the rubber in guayule occurs 

 in the parenchyma " cells " of the stem and root ; that is, in the pith, paren- 

 chyma rays, and cortex (the conjuctiva, in a word). These facts, though 

 known to a few, were first clearly stated by Ross in 1 908, according to whom 

 rubber occurs in almost all the cells of the ground-tissue in root and stem ; 

 that is, in those of the pith, parenchyma-rays, primary cortex, and also in 

 the wood-parenchyma. The leaves, he adds, contain little or none. While 

 I have been able to confirm these conclusions in general, several additional 

 details have come to light. 



Rubber occurs invariably in all the cells of the resin-canals (plate 41, 

 figs. 4-6) . While I am unable to state positively that it occurs here earlier 

 than elsewhere, it certainly is secreted most rapidly. There is, however, 

 evidence that the former statement is true. 



In the primary hadrome parenchyma rubber does not occur early. 

 In the preparations from which the photographs on plate 42 (figs, i and 2) 

 were taken, there was no trace of rubber. There can be no doubt, how- 

 ever, that rubber is secreted by some or all of these cells later on, as they 

 are replete after some secondary thickening has occurred in the leaf -trace, 

 which, of course, suffers no secondary change 2 (plate 42, fig. 6). The cells 

 of uniseriate parenchyma rays, in consonance with other parenchyma-ray 

 cells, contain rubber. 



In the primary leptome, under the conditions noted for the primary 

 hadrome in the preceding paragraph, rubber occurs at least in the fiber- 

 cells and in the parenchyma (plate 42, fig. 6). This takes place after the 

 abscission of the corresponding leaf. In secondary leptome I have seen 

 small amounts in mature fiber-cells, just before sclerosis sets in. Occur- 

 rence of rubber in these elements appears, therefore, to be a function of 

 age. It is secreted normally in leptome-parenchyma, and at the same 

 time is in adjacent parenchyma-ray cells (plate 42, fig. 4). 



In the secondary leptome rubber may also be seen in all the elements 

 of the sieve-tissue (plate 40, figs. 8, 9). It is true that the very narrow 



1 Fron and Franjois, 1901. 



2 The parenchyma of the secondary hadrome is rather scanty and of small 

 elements. They do not secrete rubber as early as the medullary-ray cells in the 

 same zone, but ultimately do so. The rubber may best be seen in longitudinal 

 sections, treated with boiling 10 per cent caustic potash and stained with alkanet. 

 The rubber then appears as small series of globules. The wood may also be macerated 

 by means of Schultz's medium, and later stained. 



12 



