Zo6 I'HK PLANT WORLD 



only of soil, the distribution of vegetation is determined by 

 the fissures. In the case of guayule we have an exception, 

 tor this plant may send out a shallow lateral root over a 

 block of stone, and shoots may start above it. Very fre- 

 quently we find plants which have grown in this position, with 

 their roots straddling the subimposed rock. Such are al.nost 

 invariably retonyos. Plant A in the figure was found so 

 placed. There are plants which can compete with the guay- 

 ule in this regard. Such an one is the lechuguilla {Aifcrcc 

 lechegiiUla Torr. ) which spreads out by means of stolons, 

 and occupies areas for itself to the exclusion of everything 

 else. Nevertheless, 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 by 

 the rate of growth and quickness with which the ability to 

 flower is reached by retonyos. The rate of growth is a func- 

 tion of the size of the mother root; but it is a Nery common 

 thing for a retonyo to grow lo cm. and to come into flower 

 in two months in summer (K and L in the figure). Seed- 

 lingson the other hand scarcely e^•er flower before the third 

 year, and the amount of growth then does not more than 

 equal that of a single stemmed retonyo. At the end of three 

 years the retonyo makes a respectable plant ( F in the figure) 

 and flowers abundantly. Ihe influence, therefore, which re- 

 tonyos would ha^■e in reforesting processes, both by their 

 own growth and by seeding, can be well appreciated, and 

 probably with difliculty overestimated. Some basis for 

 judgment in this regard will reward a study of the accom- 

 panying photograph. 



Ihe case of mariola is somewhat difierent. In the first 

 place, the root-system is tiifterent in that the laterals run at 

 a steeper angle into the soil. Retonyos are formed occa- 

 sionally, but as far as my ol)servation goes, always close to 

 the plant, within say 5 cm. What, however, always happens 

 is this: From the basal portion of the stem where there are 

 manv dormant buds, as a sequence of the short internodes 



