178 Guayule. 



elements contain very little, but this amount may be clearly demonstrated 

 in longitudinal sections treated as above described. 



In the cork-cells rubber occurs in a secondary condition as small 

 droplets, derived by the breaking up (possibly an emulsification) of the 

 compact masses in the outer cortex. These droplets are larger in cork-cells 

 on either side of collenchymatic zones which are remnants of the periclinal 

 walls of collenchyma (plate 42, fig. 8). 



Rubber is secreted in the parenchyma of the pseudoty loses (plate 42, 

 fig. 3), quite as in the adjacent cells. 



In the leaf the amount of rubber, though always small, is in propor- 

 tion to its age. In the oldest leaf I have observed, rubber occurs in drop- 

 lets in the outermost palisade-cells of both surfaces, and less conspicuously 

 in the subjacent, but usually in no other chlorenchyma cells (plate 43, fig. 

 5). It may occur only in the ventral palisade in younger leaves. In very 

 minute droplets it is to be found also in the collenchyma and endodermis 

 of the midvein and in nearly all of the non-chlorophyllous cells in the 

 region about it, and in the leptome, both in the companion and sieve cells. 

 Curiously enough, it is not to be found in the secreting-cells of the resin- 

 canals, though, on the other hand, it is in small but conspicuous quantities 

 in the subjacent cells. A minute amount occurs in the epidermis, espe- 

 cially near the midvein, and in the non-chlorophyllous cells near the smaller 

 veins. The maximum quantity, negligible from the economic point of 

 view, occurs in the oldest leaves which have passed through a drought 

 period. The material which gave these results was collected in the spring 

 of 1909 before the summer rain of that year. 



In material collected from irrigated plants at Cedros in April 1909 

 rubber may be detected in exceedingly minute quantities in the basal part 

 of the leaf only. A single minute droplet not more than one-fourth the 

 diameter of those seen in the field plant may be seen in each outermost 

 palisade-cell of the upper (ventral) surface. They are a trifle larger near 

 the midvein. In the non-chlorophyllous tissue near this the rubber may 

 also be detected in still more minute quantities. 



Since within the periphery of the wood cylinder only the conjunctiva 

 and a small amount of wood-parenchyma contain rubber, and since in older 

 wood the medullary rays (in part) and the pith and its canals are dead 

 and disintegrated, the wood cylinder contains less rubber than the cortical 

 tissues, but it is also less resinous in its primary condition. Recent work 

 by Whittelsey (1909), however, indicates that in stems of an advanced 

 age, at any rate, the amount of true rubber is practically reduced to nil, 

 though in the young twigs the proportion of rubber within the periphery 

 of the w T ood cylinder is large. We must conclude, therefore, that the 

 rubber in the older wood undergoes chemical change, and is broken down 

 into related materials. There is no doubt that some such change takes 

 place also in the secondary cortical tissues cut out by the inner periderm, 

 and this, as I have shown, is a considerable part of the volume of the 

 "bark" in older stems. 



