52 



Guayule. 



character could be traced around the ridge. Just above this zone another could be 

 made out in which the retonos were abundant and the seedlings scarce, while 

 coming to the top of the ridge the seedlings again outnumbered the retonos. Thus 

 on that part of the slope most affected by erosion, and where there is more chance 

 of uncovering the shallow roots, the retonos are most abundant. It would appear, 

 therefore, that the exposure to light is a potent, if not the most important, factor 

 in inducing budding in the roots. Yet I have found that when a plant is removed 

 by cutting at the base so as to sever the roots and leave them in the ground, shoots 

 start from the root, not only where the root is accidentally exposed, but as far 

 back as the drying out of the root makes it necessary. A root thus severed in 

 January failed to bud till June in consequence of the lack of rain; when at last it 

 rained, the buds started out 12 cm. away from the cut end and several centimeters 

 deep in the soil. On the other hand, roots purposely exposed for a portion of their 

 length and slightly wounded had failed to start buds at the end of six months when 

 last examined. 1 So the case appears to be more complicated than at first appears. 

 Injury may be a factor at times, but, experimentally, I have shown that scarring 

 or cutting the cortex is not sufficient to insure budding, at least under field condi- 

 tions, for it is probable 

 that the exposure to a 

 low relative humidity in- 

 hibits the growth of callus 

 on exposed roots. It is 

 more probable that had 

 roots been injured and 

 left covered with soil, 

 positive results would 

 have accrued. 



This occurrence of 

 retonos in guayule pre- 

 sents a very interesting 

 biological phenomenon. 

 In a habitat where the 

 rainfall is very meager, so 

 that years occur in which 

 the conditions for germi- 

 nation are prohibitive, and where, moreover, sudden and severe rains wash the soil 

 on the steeper slopes severely enough to remove seeds or expose seedlings when 

 young so as to prevent their further growth, it will easily be seen that the vegetative 

 method of reproduction presents certain very marked advantages. This is true also 

 where the soil is confined to the crevices of the native rock where it lies at or very 

 near the surface. This condition occurs very frequently in North Zacatecas, where 

 large areas will be seen in which the vegetation is confined to bands of outcropping 

 rock, where it occupies the soil beneath the edge of a stratum. Where the relation 

 of the strata to the surface is such that flat blocks of rock support but a thin layer 

 of soil, the distribution of vegetation is determined by the fissures. In the case of 

 guayule we have an exception, for this plant may send out a shallow lateral root 

 over a block of stone, above which plants may start. Very frequently we find 

 individuals which have grown in this position, with their roots straddling the sub- 

 imposed rock. Such are almost invariably retonos. Plant i (plate 9, fig. B), was 

 found so placed. There are other plants which can compete with the guayule in 

 this regard, such as the lechuguilla (Agave lecheguilla) , which spreads out by means 

 of stolons, and occupies areas for itself to the exclusion of everything else. It is 

 clear that the habit described is of no small importance in the fight for foothold. 

 One can easily imagine, too, that a distinct advantage is to be had in the rate of 

 growth and the quickness with which the ability to flower abundantly is reached 

 by retonos. The rate of growth is relative to the size of the mother root; but it is a 

 very common thing for a retono to grow 10 cm. and to come into flower in two months 

 in summer (plate 9, fig. B, 10 and n). Seedlings, on the other hand, flower only 



1 Only negative results were had as late as September 1908. 



FIG. u. Retonos, showing position of adventitious roots, pr., 

 proximal portion, and dr., distal portion of mother-root. 



