350 BOTANICAL GAZETTE [ NOVEMBER 
bowlders. The area which they covered extended about 45°" away 
from the plant, which was between 12 and 15” high. 
Cereus enneacanthus Engelm.—This rather ascending, cespitose 
form showed, as in the other cases, the two sets of roots; horizontal, 
long and sinuous; and vertical, abruptly tapering and much-branched. 
Several specimens were examined. 
ConcLusions.—In the majority of the larger Cactacee there are 
two distinct root systems—vne horizontal, for absorptive purposes ; 
the other passing downward, for anchorage. The depth of the hori- 
zontal system varies with the degree of penetration of surface water, 
which, in turn, is to some extent dependent on the character of the 
soil. One of the specimens of Echinocactus was examined the day fol- 
lowing a gentle rain which had continued intermittently for two days, 
thus giving the water time to soak in as it fell. The soil was dampened 
to a distance of about four inches below the surface, just the depth of 
the absorptive root system. 
Observations upon young plants of Opuntia fulgida give the follow- 
ing as the sequence of root formation. The first to be developed by 
joint or seedling are the vertical roots, which for a time act both for 
; ee d 
absorption and anchorage. As the plant increases In size the groun 
and in order 
developed 
onally 
appearing in part as horizontal branches of vertical roots. 
zontal system covers an area far greater than that sheltered by the parts 
above ground. As the soil is generally firm, and the horizon i 
give some support, there is little need for any roots to descend pa : 
great depth. The small amount of surface exposed to winds 
considered another reason for this. is 
Correlated with the difference in distribution of the two a eps 
a difference in structure, as may be proved bya most superficial se 
nation. The horizontal roots are in all cases extremely brittl¢ ter 
vertical much more elastic and capable of withstandin; gree j 
tension. The woody cylinder in large absorptive roots p his differ- 
relatively than that of the anchoring roots. Microscopically t ani 
ence may be most easily seen in the secondary xylem. For ae were 
tion sections of roots of Opuntia fulgida and Echinocactus Wist oe Ff 
used. The xylem of the absorptive reot is composed te of the 
ducts, that of the anchoring roots, which, after the app 
tal roots 
