38 BOTANICAL GAZETTE [juLY 
between the assimilative tissue and the guard cells. Through 
this collenchymatous tissue run canals connecting the two; these 
canals must at all times be filled with water vapor, and probably 
act in a much more effective manner than would an equally large 
external vestibule. The hypoderma is of varying thickness, of 
one cell layer only in some species of Mamillaria, of seven to 
ten in certain other forms, as in Ceréus giganteus. There appears 
to be no difference in the character of the cells in the various 
groups; the walls are irregularly thickened, the thin portions 
allowing osmotic transfer of materials between the cells, and 
from them to the epidermis. Portions of hypoderma are shown 
in figs. I—}. 
With respect to assimilative tissue, Ganong speaks of deep 
palisade layers. Schumann, on the other hand, says that the 
cells are not in palisade form, but spherical. In point of fact, 
there seems to be considerable variation in this regard. In no 
form which I have examined have I found an assimilative tissue 
of absolutely spherical unmodified cells. There has always been 
an elongation perpendicular to the surface, sometimes slight, as 
in Opuntia leptocaulis, in other cases very well-marked, as in 
Echinocactus Wishzeni. There is also a general compacting of the 
tissue in this region. These elongated cells extend inward in 
the form of more or less definite chains or filaments, grading 
rather slowly into the ordinary parenchyma tissue, which in many 
cases contains chlorophyll in some quantity, in others is color- 
less. In the latter instance the chlorophyll, as noted with the 
naked eye, seems confined to a well-marked rind. 
It is perhaps only fair to suggest that environment may have 
a considerable influence upon the development of the palisade 
form in this family, as well as in those which possess functional 
leaves. The classical works of Stahl, Pick, and other investi- 
gators leave little question but that the elongation of the assimi- 
lative tissue, at least in some degree, is in direct response to the 
stimulus of light. The habitat of the plants here described and 
contrasted is one marked for its great light intensity, which, as 
I have found from a rather comprehensive examination of foliar 
