Igor] 
structures, leaves its imprint 
upon the arrangement and de- 
velopment of assimilative tis- 
sue in almost all cases. Schu- 
mann’s statement that the 
assimilative cells are spherical 
may be true of the plants when 
a general average is taken, in- 
cluding epiphytic types, and 
others growing under entirely 
different light conditions. Pos- 
sibly also the effects of arti- 
ficial cultivation may have some 
place in his results. 
The medulla in many cases 
is of thick-walled and pitted 
cells, as Ganong describes, but 
this is by no means universal, 
often varying within the limits 
of a genus as now established. 
The cells are often exceedingly 
thin-walled and have no need 
of pits to maintain connection 
between them. The presence 
in this tissue of mucilage, crys- 
tals, etc., is likewise variable 
with the species, and needs 
separate discussion. Not only 
do the crystals vary in distribu- 
tion to a wide extent, but also 
in relative frequency, some 
Species being far richer in this 
deposit than others of the same 
habitat, a fact which, it seems 
SOUTHWESTERN CACTACEAE 
Fic. 1.—Epidermis and hypoderma of 
Opuntia arbuscula; opening of stoma does 
not show, owing to slightly oblique charac- 
ter of section. 
Fic. 2.—Epidermis and hypoderma of 
Cereus Fendleri in section, showing stoma. 
Fic. 3.—Hypoderma of Cereus Fendleri, 
seen from inside, showing character of walls 
and opening of stoma canal (s). 
to me, can only be referred back to the varied selective power 
of roots for chemical solutions. 
