56 Guayule. 



the left-hand plant in the same figure show extremes of rate of growth, 

 somewhere between which lies the average, which it is desirable to know 

 in estimating the rate of reproduction. 



A still further difference in habit, which is not very readily distin- 

 guished from the foregoing at first glance, is one recognized by persons 

 engaged in the gathering of the shrub, who designate the two types in 

 question "macho "or male and"hembra"or female. The differences, which 

 are shown in plate 14, fig. B, were pointed out to me by Don Jose' Herrera, 

 a gentleman who has had a great deal of practical experience in collect- 

 ing shrub. " Macho " guayule has fewer branches, and they have a larger 

 diameter than those of the "hembra,"in which the branches are much 

 more numerous. These terms are not here used in the sense spoken of on 

 page 4, to distinguish guayule from mariola, which latter is sometimes 

 called "hembra de guayule," but merely to designate the plant with the 

 stronger and therefore "macho " habit and that with the weaker or " hem- 

 bra" habit. These adjectives are used analogously with respect to other 

 plants showing similar differences. " Hembra " guayule makes greater bulk 

 when made up into bales, and for this reason those who gather shrub pre- 

 fer to take it if they are being paid at a rate per bale. Whether the dif- 

 ferences are biotypic or are due merely to environmental conditions can 

 not be said; nor whether there are other correlated differences, as in the 

 amount of rubber secreted, though such are variously claimed to obtain. 

 There appears to be a stronger tendency in the " hembra "for the branches 

 to run out into inflorescences, entailing a greater amount of dying back 

 at the close of each growing-season, and thus it may turn out that these 

 differences are essentially the same as those mentioned previously. 



Finally, many guayule gatherers and others think to recognize dif- 

 ferent kinds as to color-characters, either of the bark or of the leaves. In 

 Durango white guayule ("bianco") is distinguished from dark or " prieto," 

 though no other characters could be pointed out to separate the two 

 kinds. Indeed, when a branch was exposed to view in one position, so that 

 the under surface of the twigs was seen, it was pronounced "prieto," 

 and when the upper surface of the same branch was later shown it was 

 called "bianco." This color difference, as between the upper and lower 

 surfaces of the branches, is quite constant. 



"Blanco" and "ceniso" or ashy guayule are maintained to be dif- 

 ferent also, though the same difficulty of seizing upon other than mere 

 color differences obtains. So far as I could determine, "ceniso" guayule 

 was shrub which had been exposed to severer drought, in consequence 

 of shallower soil in exposed positions, as on benches, and in which the 

 leaves had therefore dried to a dirty-yellowish color. Prolonged study 

 might , however, discover that some of these differences are constant and 

 racial, and the matter therefore deserves more consideration. 



SIZE. 



The question is often asked, especially by persons interested from 

 the business point of view, as to the size which the guayule attains. It 

 may at once be said that anything like the maximum size is a matter, 

 or will be shortly, of academic rather than economic interest. Once the 



