194 Guayule. 



tion and amount of growth, becomes, in effect, a tap-root of the branch 

 from which it springs. Subsequent development of roots of the second 

 and higher orders results in the ultimate elaboration of a complete root- 

 system. 



We find furthermore that, while the caliber of the basal-lateral stem 

 increases with age, the isthmus of tissue between this and the chief stem 

 increases only slowly, so that there is never more than a weak connection 

 established, and this ultimately becomes disintegrated. In this manner 

 the basal branches in question are set free from the parent stock. There 

 results, therefore, from a single original stock, a group of independent 

 individuals closely crowded together. 



A departure from this behavior is sometimes to be found. A glance 

 at the root-system of a single stock will show that the lateral roots run ob- 

 liquely into the soil, so that they soon attain a considerable depth. From 

 the upper portion of these lateral roots retonos occasionally arise which 

 behave much as do the basal-lateral branches above described, and the 

 net result is the same, namely, to produce a crowded group of individual 

 plants. 



The root-system of the guayule, on the other hand, consists of a 

 strong tap-root and several strong laterals, which arise at a short distance 

 below the surface of the soil (plate 9, fig. A). These follow a horizontal 

 path for a distance, it may be, of 2 meters or more from the plant, and con- 

 stitute a water-collecting system by which the plant derives water from 

 rain-water which does not penetrate deeply a feature shared by many 

 desert plants (Cannon, 1911). These shallow roots frequently produce 

 root-shoots (retonos) at various distances from the parent stock. I have 

 found them at a meter distant, and it is likely that they may arise still 

 farther away, though I believe less often than at shorter distances. 



It may be presumed that shoots, arising, as they not infrequently do, 

 from the basal portion of the main axis, may occasionally strike root as in 

 the mariola. Many thousands of plants, however, have been examined, 

 and only one or two cases have been found which may be permitted this 

 interpretation. We may therefore regard the method described as the 

 only normal method of vegetative reproduction under natural conditions, 

 though it has been observed to occur in the field (Station 5) in two cases 

 in which the aerial portion of the plant had been removed. 



On observing for the first time the conditions above described in the 

 mariola, it occurred to me that it ought to be possible to induce the guayule 

 to behave similarly. The fact that a guayule retono strikes new adven- 

 titious roots from its basal zone (fig. i'i), and that this, in common with 

 that part of the chief axis above the cotyledons, has a different ana- 

 tomical structure from other stems, gave color to the notion that there 

 are physiological grounds for entertaining the belief that, with proper 

 treatment, the possibility might be realized. 



As experiments to this end would have necessarily involved a long 

 period of time, it was fortunate that I had under observation at Cedros 

 plants which had been growing for the major part of two seasons under 

 irrigation. This was in September 1908. These plants had grown from 

 stumps which were planted in March 1907, by Mr. C. T. Andrews. The 



