always contains fragments of minute fibers, single or in bundles, and. together with other fragments of cell walls, enable one to determine the origin of the rubber microscopically.
From the inner limit of the cortex, extending toward the center, are radial, wedge-shaped masses of wood (w, Fig. 9). This is composed of mechanical tissues with water vessels scattered irregularly 
Fig. 9. Semi-diagrammatic Sketch of Two-year-old Guayule Stem in Transverse Section. throughout it. These may be identified by the width of their lumina. At the inner ends of the wood masses may occur additional bundles of "bast" fibers.
The central area is occupied by the pith composed (p in the figure) of cells identical in every detail except in shape with those of the cortex. The pith cells are nearly circular in outline. The)', too, contain rubber. Extending from the pith to the cortex and separating the wood bundles from each other, as also the bast bundles, are the so called medullary rays (mr). Their cells, flattened tangentially, are otherwise quite like those of the cortex and pith, are equally rich in rubber. The pith, again like the cortex, is traversed longitudinally by resin canals. A point of interest here is that the canals with age become plugged by ingrowing masses of tissue, thus in a measure preventing the leakage of resin, especially in the cortex when invaded by cork. In the pith there is an undoubted downward leakage of resin, which, infiltrating into the older wood below, may be detected by chemical analysis. It is thus evident that the resin content of guayule wood is accidental.
By subsequent growth, year by year additional layers of bast and wood are laid down at the inner limit of the cortex, where the cambium, or actively growing tissue, is situated (c, Fig. 9). In the bast, and here alone, new resin canals are formed, in radial rows, therefore.
From this very curtailed description, it appears that, in the guayule, the rubber occurs as droplets—really very minute—each enclosed in a sac of albuminous material (protoplasm), this again surrounded by a sac of cellulose (the cell wall). All the cells of the pith, medullary
