182 Guayule. 



THE STEM. 



In the stem, the first evidences of rubber are to be observed in the 

 secreting-cells of the cortical and medullary canals simultaneously. The 

 dark appearance of these cells in figure i , plate 42, is due, in part, to their 

 larger rubber-content, but in part to the denser protoplasm. The condi- 

 tion to be seen in these cells is represented by the camera drawing in plate 

 31, figs. 10 and 12. The section was taken toward the apex of a newly 

 grown twig of a field plant, collected on July 22, 1908, and was then about 

 six weeks old. In all the cells of the conjunctiva very minute granules of 

 rubber could be seen, but not more in the cells near the canals than else- 

 where. In the stem, therefore, secretion appears to begin first simultane- 

 ously in the canal-cells of the pith and cortex, and then in the conjunctiva. 

 It is, however, quite readily determined that the physiological activity of 

 the pith is greater than that of the cortex. In fig. 2, plate 42, is shown 

 a section taken from the twig just mentioned, but near the base of the 

 new growth. One or two peridermal divisions have ensued, while other 

 secondary changes may be noted. The rubber-content of the pith-cells is 

 obviously greater than that of the cortex in this section. Further, I have 

 noted in irrigated plants that the amount of rubber is greater in the outer 

 than in the inner cortical cells (plate 43, fig. i). It seems, therefore, that 

 the deportment of both root and stem is essentially the same and that the 

 hypocotyl, though possessing a pith, behaves as the root. 



During secondary thickening, as in the root, the secondary cortical 

 canals exhibit early activity in rubber secretion, while this is taken up by 

 the oldest parenchyma-ray cells first, simultaneously, therefore, at the 

 inner and outer edges. 



THE LEAF. 



In the leaf the earliest appearance of rubber is in the outer palisade 

 in the ventral moiety. I found rubber in these cells only in old leaves of 

 irrigated plants. The analogy with the condition described for the stem, 

 in which superior activity is shown by the pith, is clear. But the failure 

 of the leaf-canal cells to show greater activity than the neighboring con- 

 junctiva detracts from the force of the comparison. The leaf observed 

 by me to be most richly supplied with rubber contained a single drop- 

 let, with a diameter about half the transverse diameter of the cells, in 

 each palisade-cell toward the median vein. The amount of the rubber 

 became less and less toward the margin. This was true also of the outer 

 palisade of the dorsal (lower) surface, and in a less degree of the inner 

 palisade. 



Minute granules occurred also in all the non-chlorophyllous cells, 

 mechanical and conjunctive, forming the midrib, excepting the vascular 

 and sieve elements. It would seem, therefore, that, roughly speaking, the 

 midvein is the center of rubber secretion, which proceeds through the 

 lamina toward the margins; further, that activity is shown first by the 

 outer palisade-cells, then by the inner, and first by the ventral and later 

 by the dorsal. In this regard, as already said, the analogy to the stem is 

 clear. 



