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Page:Popular Science Monthly Volume 81.djvu/325

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thin slice taken transversely through a smaller branch will enable us to gain some impression of the structure and especially of the manner of occurrence and distribution of the rubber. A twig about a year old taken at the end of a prolonged drought period will serve our purpose.

Referring to Fig. 9, the reader will note that several concentric zones of tissue appear. The outermost is cork (ck), which contains no measurable amount of rubber in small branches. In old thick stems the formation of cork in the deeper layers of the next occurring zone, the cortex (cr in the figure), results in the cutting out of rubber-bearing tissue, accompanied by a degeneration of the rubber itself, and its consequent loss. The cortex comes next. It is made up of cells of uniform size and shape, containing chlorophyll, and hence green in color. In each cell appears a large droplet of rubber, occupying nearly the whole of the interior space (Fig. 9). Here and there are oval openings which are sectional views of so many canals (rc) which traverse the cortex longitudinally. These contain a pale yellow resin, a commonly occurring material among the Compositæ. Any slight wound of the cortex, however caused, may open one or more of these canals, and the resin may then escape. Hardened, limpid drops of this, either clinging to the branches or fallen on the ground beneath, are always to be seen. The resin appears to be of little economic value, as it is soft. During the mechanical extraction of rubber, it becomes mixed with upwards of 20 per cent, resin.

Inwardly the cortex is broken up by radially placed masses of tissue partly composed of hard "bast" fibers (bf. Fig. 9). These correspond to the fibers of the flax in position and origin. Crude guayule rubber

PSM V81 D325 A fine source of guayule near las caopas zacatecas.png

Fig. 8. An especially fine stand of Guayule near Las Caopas, Zacatecas, Mexico.