The Origin and Occurrence of Rubber. 181 



terecl. That these vacuoles contain various substances in solution in the 

 inclosed water can not be doubted, and it seems likely that among these 

 are enzymes 1 which may act upon the rubber after extraction by the 

 mechanical processes in vogue. It also seems likely that the protoplasm 

 of the cells becomes intermingled with the rubber during extraction, ren- 

 dering it more or less albuminous and liable to give off the products of the 

 decay. 



CENTERS OF SECRETION. 

 THE ROOT. 



With certain exceptions, the secretion of rubber both in the stem 

 and the root, including the hypocotyl, appears to proceed from definite 

 centers. This is exemplified with especial clearness in the root, where, in 

 the cortex, the secreting-cells of the resin-canals 2 are the first to show the 

 presence of granules of rubber (plate 41, fig. 6). It is argued that secre- 

 tion actually begins earlier in these cells because the surrounding cortical 

 cells, primary on the outside, secondary on the inside, contain, at an early 

 stage of secretion, less and less rubber, as one proceeds farther from the 

 canals. The figures of plate 40 illustrate this advance in secretion, the 

 beginning of which is seen in a young stage in the development of the 

 root (plate 23, figs. 3,7; plate 40, fig. 1). If the rate of growth has not 

 been too rapid, so that a part of the primary cortex has had the necessary 

 time to secrete rubber before being cast off, the activity of secretion is 

 seen to be taken up successively by the cells further removed, until the 

 whole tissue becomes loaded (plate 40, figs. 2,3). The greater amount of 

 rubber, however, is evidently held by the cells nearer the resin-canals. 

 In the hypocotyl the same physiological relations hold. 



The secretive activity of the secondary cortex is taken up, aside from 

 those cells in the neighborhood of the canals, by successive layers of cells, 

 beginning on the outside. With the appearance of the secondary resin- 

 canals, however, a superior activity in rubber secretion in their secreting- 

 cells is to be early noted. 



On the other hand, simultaneously with the appearance of rubber in 

 the primary canal-cells, it appears also in the innermost cells of the paren- 

 chyma rays, the function of secretion being taken up successively by the 

 next outer cells, and so on. This is apparent in the figures (plate 40, figs. 1 

 to 4). If a period of rapid growth follows one of stasis, the newly formed 

 parenchyma-ray tissues will show an entire absence of rubber (plate 40, 

 fig- 3)- When secretion again begins, it starts simultaneously in the 

 outermost and innermost cells of the parenchyma ray. 



THE HYPOCOTYL. 



In the hypocotyl a similar condition prevails, though here, as in the 

 definitive stem, there is a pith. That is, the innermost parenchyma-ray 

 cells assume secretive ability earlier than the pith-cells, which is not true 

 for the definitive stem (plate 40, fig. 5). 



1 The presence of oxidases in extracted rubber, both in latex rubbers (Spence, 

 1909) and in guayule rubber, is known. 



2 In view of the emphasis which has been placed by many writers on the endo- 

 dermis as seat of high physiological activity, the beginning of the secretion of rub- 

 ber in the resin-canal cells, which are constituents of the endodermis, is of very great 

 interest. 



