ROOT-SYSTEMS OF PERENNIALS. 57 



mental modification can be shown by the main characters of the root-system 

 of a larger specimen. A sahuaro 6.8 meters high and bearing 4 branches, 

 from 40 cm. to i meter in length, and growing in a situation apparently 

 similar to that of the cactus above described, had a relatively heavy root- 

 system. The roots of the anchoring system were sharply differentiated 

 from the superficial roots and were composed of a cluster of 10 main roots 

 with their branches which ran directly downward about 77 cm., and ended 

 abruptly on reaching the bed rock. Of the superficial system three leading 

 laterals were uncovered, one of which ran southeast over 9.7 meters, one 

 northeast over 4 meters, and one west over 4.5 meters. All of these roots 

 were verj- heavy, especially at the base, as plate 15 indicates. In this, and 

 in other specimcms examined, it is clear that the increased diameter of the 

 superficial roots is connected with the large size of the sub-aerial portion of 

 the plant and its need of better support than would be afforded by the 

 "anchoring" roots alone. A proof of the importance of the laterals as 

 mechanical supports of the stem is often afforded by the spectacle of 

 ujDrooted plants, whose laterals have become diseased and no longer func- 

 tion as efficient props or stays. 



A specimen of sahuaro growing on the flood-plain of West Wash was also 

 examined. The root-system did not differ materially from either of those 

 above described ; that is, there was an anchoring portion and a superficial 

 portion, which in the plant seen (which was less than 2 meters high) were 

 not as yet sharply differentiated either in function or in position. The main 

 root did not penetrate deep, although the soil was somewhat deeper than 

 at either of the other habitats mentioned. This was the leading point of 

 interest : the root-system of the sahuaro is essentially superficial even if the 

 soil conditions are such as to permit deep penetration. 



From the brief discussion of the root-system of the sahuaro it will be seen 

 that there is no sharp distinction in function between the superficial roots 

 and the anchoring roots, as is to be found in Echinocadus especiall}', but 

 that as the plant increases in size the superficial system gradually assumes 

 the role of supporting the sub-aerial portion in addition to its chief role 

 in the absorption of water. 



In the immediate vicinity of the cacti growing on the bajada were also 

 the following perennials : Echinocereus fendleri, Covillea tridentata, Encelia 

 farinosa, Kramcria canesccns, Parkinsonia micro phylla, and some other 

 plants, especially species of cacti. 



The relation of the roots of sahuaro to those of other perennials was exam- 

 ined closely only in connection with the study of the sahuaro which was 1.2 

 meters high. In this case one specimen of Parkinsonia and one of Covillea 

 were situated within the proper root-area of the sahuaro, as plate 14 shows. 

 The Parkinsonia was growing 1.3 meters distant from the stem of sahuaro. 

 The following were the leading points in the character of its roots. There 

 was a tap root which went straight down to the caliche, where it terminated 



