ROOT-SYSTEMS OF PERENXIALS. 49 



which arose from these vertical roots and extended in a more or less horizon- 

 tal direction for various distances not far beneath the surface. There was 

 no well-defined tap root, or supporting roots, all roots functioning mainly 

 as absorbing roots. 



PiQ 4 — Opunlia discata. Ilnrizontal and vertical extension of root-system. 



The main points are shown in fig. 4, which represents the horizontal as 

 well as the vertical extension of the root-system. For the most part the 

 roots are confined within a radius of 50 cm. from the central axis, but two 

 laterals, the one going uphill and the other down, were approximately 1.5 

 mm. in length. The root reaching southward maintained a depth of 20 cm. 

 for a distance of 20 cm. and then ascended rapidly until it lay at the 10 cm. 

 level, about which it remained during the rest of its course. The roots 

 extending east were approximately 15 cm. deep. Those to the west ran 

 along the face of a boulder at a depth of 15cm., and after leaving this protec- 

 tion quickly rose and occupied a position near the surface. The roots of the 

 Opuntia were slender throughout their course and branched very little. 



The roots were seen frequently to be in intimate association with boulders 

 whose presence and position greatly affect the branching and probably 

 other habits of the root-system. Where the rocks are shallow the roots run 

 beneath, but where they are as deep as 25 cm., for example, they run along- 

 side. The roots which are placed under rocks are closely appressed to them 

 and form uitricate patterns as they there branch very densely ; such roots 

 also remain functional for a longer period than those of the same plant less 

 advantageously situated. These features are clearly to be associated with 

 the better water relations that are obtained beneath the rocks which serve 

 to retain as well as to conduct water better than the bare soil. As illus- 

 trating this statement the following observation on the penetration of water 

 may be noted. Soon after a rainfall of half an inch, by which the soil would 

 usually be moistened to a depth approximately 8 to 1 2 cm., that underneath 



