184 Guayule. 



examined Aug. 14. At the base the rubber in the pith was finely granu- 

 lar, showing in addition a tendency to agglomeration (plate 42, fig. 2) ; in 

 the extreme inner and outer cells of the parenchyma rays the rubber was 

 very finely granular, while in the cells lying on either side of the cambium 

 there was none or extremely little; in the primary cortex it was finely 

 granular, but was in somewhat larger granules in the secondary cortex; 

 large granules occurred in the younger resin-canal cells (in the secondary 

 cortex) and agglomerated masses in the older canal cells (in primary 

 cortex and pith) . Near the apex of the stem the rubber was found only 

 in extremely minute granules everywhere (plate 42, fig. i) excepting in the 

 resin-canal cells, where they were somewhat larger, but still small (plate 3 1 , 

 figs. 10 to 12). 



4. A similar twig, examined Sept. 8, showed that the condition seen 

 at the base in the twig described immediately above had advanced toward 

 the apex about one-third the length of the twig. At the base the rubber 

 had increased till it had become coarsely granular, except in the paren- 

 chyma-ray cells nearer the cambium, in which it was still finely granular. 

 Five mm. from the apex there was still scarcely sufficient rubber to be 

 observable, except in the resin-canal cells. 



I was unable to obtain material during the succeeding few months, 

 so was prevented from following the march of secretion after September 8. 

 It is, however, clear that the rate of secretion is so slow, as compared with 

 the rate of growth, that for at least four months after the beginning of the 

 rainy season the new parts contain only very small quantities of rubber. 

 From this time on the secretion of rubber probably proceeds more rapidly, 

 but it is still to be determined when the maximum is reached. This is a 

 point of very great importance. 



5. Turning to irrigated plants, I need cite the evidence from only 

 three examinations: 



(a) A branch (plate 2 1, figs. A, B) of a Cedros plant (plates 4 and 17, fig. 

 B) which began to grow in 1907 and was examined in August 1908. In 

 examining the 1907 growth no rubber was detected in the pith, probably 

 because the small amounts secreted in 1907 had disintegrated; the older 

 cells (of 1907) in the parenchyma rays contained rubber in fine granules 

 near the cortex; in the cortex and resin-canal cells there were coarse 

 granules with more or less agglomeration. The new tissues of 1908 con- 

 tained only very minute granules. In the 1908 growth, near the base, the 

 rubber was visible in very fine granules, save in the primary cortex, where 

 there was none; in the resin-canal cells coarse granules, these still larger 

 in the pith-canals; midway between the base and apex there were very 

 fine granules of rubber in the pith and parenchyma rays ; the resin-canal 

 cells had coarse or agglomerated granules ; fine granules were visible in the 

 secondary cortex, but none in the primary. Four centimeters from the 

 apex, where the stem was still herbaceous, minute granules of rubber 

 had appeared only in the pith and inner parenchyma-ray cells nearby ; 

 it was present in coarse granules in the resin-canal cells of the pith, and 

 in fine granules in those of the cortex ; the cortex itself contained none 

 (plate 43. % i). 



(b) A branch from a single Cedros plant collected in April 1909 (plate 

 17, fig. B), after a prolonged drought extending with practically no inter- 



