1917] MARKLE—ROOT SYSTEMS 195 
and taper very slightly and bear numerous small lateral branches, 
especially near their distal ends. Older individuals probably have 
root systems more extensive than that of any other plant of the 
arroyos Or mesa. , 
Opuntia fragilis.—This is the smallest and most common cactus 
of the mesa, where 
large colonies form 
mounds of sand or 
adobe. The root 
system is very 
superficial and con- 
sists of one or two 
Main roots with 
numerous small 
lateral branches 
(fig. 23). 
Opuntia arbor- 
escens.—This is the 
only large cactus 
found in the region. 
mountains and 
‘occurs sparingly 
See ek a dae ee Ok ie 
specimen shown in ia rae e 
figs. 24 and 25 
grew in the latter situation and was only 2.5 ft. i 
root system is similar to the type described by ( 
larger forms occurring near Tucson. There i is a shar 
tion of absorptive and anchorage roots. The forn 
and thin and occur within an inch or two of the 
