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THE POPULAR SCIENCE MONTHLY 



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 com- 

 posed of mechanical tissues with 

 water vessels scattered irregularly 

 throughout it. These may be iden- 

 tified by the width of their lumina. 

 ">£§^£S^-5^I^^s^? At the inner ends of the wood 



masses may occur additional bun- 

 dles of " bast " fibers. 



The central area is occupied by 

 the pith composed (p in the figure) 

 of cells identical in every detail ex- 

 cept in shape with those of the cor- 

 tex. The pith cells are nearly cir- 

 cular 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 (rar). 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 be- 

 come plugged by ingrowing masses of tissue, thus in a measure prevent- 

 ing 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 



Fig. 9. Semi-diagrammatic Sketch 

 of Two-year-old Guayule Stem in 

 Transverse Section. 



