Description of the Guayule. 55 



spurs, which, because of the more rapid growth of the shoot in mariola, 

 are more numerously developed. The transition into the peduncle is grad- 

 ual, and not sudden, as in guayule; this organ is, therefore, not sharply 

 delimited, either morphologically or anatomically, and is leafy and pro- 

 vided with buds well up beneath the inflorescence. In the following grow- 

 ing season, and this usually means in the following year, some of the 

 short spurs develop into leafy branches and in their turn terminate in 

 peduncles. These, like all the branches, are slender and tapering, and 

 their position, rate, and manner of growth result in a close interweaving 

 of stems, in striking contrast with the guayule. 



BIOTYPES. 



Returning to the subject of habital types in the guayule, it has been 

 found that some plants have the mariola manner of growth (plates 12 

 and 13). Instead of an abrupt termination of the stem at the base of the 

 peduncle, the transition is gradual and the stems are of smaller diameter 

 than in the usual type. Foliar differences are to be noted beyond. The 

 matter is possibly of practical importance, as the slender branches with 

 vaguely delimited flower-stalks would, mutatis mutandis, contribute to pro- 

 duce a less desirable form of plant from the point of view of production. 

 A phylogenetic interest also attaches to it, inasmuch as the mariola habit 

 is more closely comparable to the herbaceous manner of growth, as dis- 

 played by congeneric herbaceous species, than is the guayule habit. On 

 this score, as on others, the guayule is the type more widely divergent 

 from the theoretical herbaceous ancestor. 



These differences are, indeed, quite fundamental, and may be traced 

 back to the earliest seedling stages (plate 13). The clearness of the dis- 

 tinctions is such as to indicate that we are dealing with a field mutant, 

 and the differences in the structure of the awns (pappus) would seem suffi- 

 cient ground, in the light of the taxonomy of the genus, to warrant us in 

 regarding the broad-leafed type as a distinct species. The two forms, 

 P. argentatum proper and this closely related form, be it a well-marked 

 species or a type of less taxonomic evaluation, are remarkably distinct, 

 and call to mind many similar juxtapositions of closely related species, 

 recognized as such, known to occur among plants, but not yet properly 

 appreciated as evidence in the discussion of isolation (Lloyd, 19056). 



Another difference in the habit though not correlated, it appears, 

 with the manner of development of the inflorescence is seen in what 

 may be termed straight and crooked limbed forms. The one is clean- 

 cut and smooth-limbed, each span of growth being nearly straight; the 

 other is rougher barked, the more slender limbs showing marked curva- 

 tures. The former is the more rapidly growing type, suggesting differ- 

 ences in the available water-supply. One frequently finds examples of 

 very marked growth differences in field plants, such as are shown in 

 plate 9, fig. A, of which the right-hand plant grew in a shallow rock crev- 

 ice and was unable to develop a competent root-system. The annual 

 accretions of growth in this plant were very short, not exceeding a centi- 

 meter, and this resulted in the production of a very dense, much-branched 

 mass of limbs, as seen in plate 9, fig. A, on the extreme right. This and 



