(U'.Wri.K AXn MARIOI.A. 20; 



abundant, and the seedlings scarce, while coming to the top 

 of the ridge, seedlings again outnumbered the retonyos. Thus 

 on that part of the slope most affected by erosion and where 

 there is more chance of uncovering the shallow lying roots, 

 the retonyos are most abundant. It would appear therefore 

 that exposure to light is a potent if nut the most important 

 factor in inducing budding in the roots, "let I ha\'e found 

 that when a plant is removed by cutting at the base so as to 

 sever the roots and leave them in the ground, retonyos start 

 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 it at last did rain, the buds started 

 out I 2 cm. away from the cut end, and several cm. deep in the 

 soil. On the other hand, roots purposely exposed for a por- 

 tion of their length failed to start buds for six months, when 

 last examined. !r'o that the case appears to be more compli- 

 cated 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 ensure budding. 



I his occurrence of retonyos in guayule presents a \ ery 

 interesting problem biologically. In a habitat where the rain- 

 fall is very meagre, and where years occur in which the con- 

 dition for germination are prohibitive; and where moreover 

 the sudden and severe rains wash the soil on the steeper 

 slopes severely enough to remo\e 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 cre\ices of the native rock 

 where this lies at or vevy near the surface. This condition 

 occurs very frequently in Xorthern Zacatecas, Mexico, where 

 large areas will be seen in which the \'egetation is confined 

 to /ones of outcrop, occupying the soil beneath the edge of 

 a stratum. Where the relation of the strata to the surface 

 in position is such that flat blocks of rock support a thin layer 



