ROOT-SYSTEMS OF PERENNIALS. 45 



The general soil conditions of Tiiniamoc Hill have already been described 

 and need not be repeated here. It will only be necessary to add that, at the 

 precise spot where the specimen of Echinocactus was growing, the bed rock 

 was so deep that it was not encountered in the course of the excavation of 

 the plant's roots. The roots were wholly confined to the upper soil layer 

 or the malpais, which here was relatively heavy. In West Wash (the soil 

 conditions of which are also described above) the soil is somewhat lighter, 

 owing to an admixture of sand, and exceeds 2 meters in depth. The water 

 and the temperature relations of the habitats are unlike, and as far as 

 studied are also characterized in the section above referred to. 



The Echinocactus examined on Tumamoc Hill, April, 1907, was 38 cm. 

 high and of about the same diameter. As plates 9 and 10 and fig. 2 indicate, 

 the root-system of the plant has a very meager anchoring portion and a very 

 extensive absorbing portion. The main root went straight down 20 cm. 

 and gave off several short laterals between its tip and its crown, which 

 latter was 10 cm. beneath the surface. The most deeply placed of the short 

 laterals extended 12 cm. away from the main root axis and attained a depth 

 of 25 cm. 



The main absorbing system consisted of three laterals of the first order 

 which left the tap root at its crown and extended in two directions, namely, 

 uphill and downhill. The larger portion was that which extended up the 

 hill. The uphill portion originated from a single lateral, as shown in the 

 figure, and consisted of numerous roots of the second, third, and fourth 

 orders, which were so disposed as to cover the ground included by them 

 fairly thoroughly 



The roots of the main absorbing portion of the system were of a light 

 brown color, and were rope-Hke and tough ; they were slender throughout 

 their length. The depth of the roots may be learned from the following 

 measurements, which were made on the portion which was placed uphill 

 from the main axis of the plant. At a distance of 40 cm. the lateral was 

 3 cm. deep ; 3 meters distance it was 3 cm. ; and at 3.5 meters the roots were 

 1.5 cm. beneath the surface of the soil. Thus the roots of the absorbing 

 portion of the system are all extremely shallow, so shallow that they are 

 often exposed by the eroding effects of severe storms. 



In order to observe the possible range of variation of the root-S3^stem of 

 Echinocactus, other specimens were examined, one of which was growing 

 by West Wash. The leading points in the study of this plant may be sum- 

 marized as follows : The horizontal system was sharply differentiated from 

 the vertical system, both of which, particularly the latter, were densely 

 branched. The horizontal portion of the root-system lay for the most part 

 within 5 cm. of the surface. It was apparent that the anchoring roots were 

 also very actively engaged in absorption, and that the absorbing roots were 

 relatively not so important as the corresponding roots in the specimen from 

 Tumamoc Hill. 



