Description of the Guayule. 51 



zontal position, and in this direction traverse considerable distances from 

 the plant. This condition is closely analogous to that in the guayule and 

 serves to make even clearer the significance of the arrangement in that 

 plant and in others, in all of which the tap-root system, while quantita- 

 tively inferior both as regards the number of branches and the amount of 

 water absorbed into the superficial system, may nevertheless be of a good 

 deal of importance in enabling the plant to withstand prolonged drought 

 when the shallower portions of the soil become very dry. This is indi- 

 cated by the readiness with which retonos arise from the tap-root after 

 the plant and lateral roots have been cut away. 



RETONOS. 



From these shallow lateral roots there frequently arise new adventi- 

 tious shoots, sometimes singly, sometimes in groups of two or more (Lloyd, 

 igoSc) (plate 9). They are locally called "retonos," though this term is 

 not always used strictly , and may apply to shoots arising from stem tissues. 

 It is the same word as " rattoon," used by sugar-cane planters, but as this 

 is used constantly to indicate offshoots from the base of the stem, it is 

 inapplicable as an equivalent of retono. Since it is the only term used in 

 Mexico for these shoots of root origin and as our common English equivalent 

 is characterized chiefly by its inelegance, we shall venture to retain the 

 Mexican-Spanish expression. 



Retonos usually arise from the plant at a distance of 20 cm. or more. 

 They have been found at a meter's distance, and doubtless may occur 

 still further away. The point of origin may be above, below, or at the 

 side of the root. As growth proceeds the proximal part of the root fails 

 of further secondary thickening, or at most undergoes very little thick- 

 ening. It ultimately becomes abstricted by decay, apparently induced 

 by pressure of the tissues of the retono, and quite soon loses its physio- 

 logical value. The distal portion, however, thickens more rapidly, keep- 

 ing pace with the growing retono, and takes on the proportions of a tap- 

 root, though it may always be distinguished from a true tap-root by its 

 curvature and position in the soil. Secondary, adventitious roots (fig. n) 

 later arise from the basal portion of the stem of the retono, thus amplify- 

 ing the root-system. A large root-system thus developed is shown in the 

 central and largest plant in plate 9, fig. B. 



The author of this publication stated as follows in a previous paper : 



The formation of these new plants in this manner is not spasmodic or excep- 

 tional, nor are they fugitive in their nature. Under certain conditions they are pro- 

 duced in such numbers as to entirely overshadow the numbers of seedlings ; and they 

 as frequently grow into maturity, producing a plant which, if the origin were not 

 known, would not unlikely be considered a varietal type, in point of habit. The 

 mature plant which had its origin as a seedling has a single'trunk, usually 10 cm., 

 sometimes 20 to 30 cm., in length; the mature plant produced vegetatively has usu- 

 ally a very short trunk, or a group of separate ones, more or less coalesced by growth, 

 though marked exceptions may occur (plate 9, fig. C). 



The ratio of the number of new plants arising as seedlings and of those arising 

 as root-shoots varies with the habitat. Both forms may be found in any situation, 

 but the retonos are much more numerous on stony slopes, often outnumbering the 

 seedlings. The reverse relation is seen in more level places. Thus, at the foot of a 

 low ridge I have found seedlings plentiful, as many as 30 in a square foot (these 

 small and larger ones as well scattered about relatively thickly). A zone of this 



