901] SOUTHWESTERN CACTACEAE 37 
addition of adaptive modifications. This is true particularly in 
the first form examined, Cereus Fendleri, of cespitose habit, and 
one of the most succulent. Upon cutting a shoot transversely 
one will notice that the bundles recede into the tissue for some 
distance, both in the severed portion and in that remaining on 
the plant, while the parenchymatous tissue still remains turgid. 
It is thus easily demonstrable that the bundles are normally under 
great tension. The mechanical arrangement by which such elas- 
ticity is rendered possible will be found in the structure of the 
xylem. Another point of interest, probably of value upon sys- 
tematic grounds, is the behavior of the bundles in the matter 
of secondary medullary ray formation, development of inter-, 
fascicular cambium, and anastomosis. 
Before considering the various forms separately, there are 
certain points of structure which can best be mentioned in gen- 
eral. Schumann, both in his monograph and in Engler and 
Prantl’s Pflanzenfamilien, gives an idea of the ordinary structure 
of the group. A condensed account is also given by Ganong? 
which I take the liberty to quote asa basis for the following 
discussion, 
As to the tissues, it is enough here to say that the characteristic xero- 
philous appearances are a strong cuticle, thick epidermis, perfect cork, 
sunken stomata, collenchymatous hypoderma, deep palisade layers, great 
development of pith and cortex, which consist of large, round, splendidly- 
pitted water-storing cells, often containing mucilage; a fibrovascular system, 
in general simple in its make-up, lacking annual rings, composed as to its 
xylem of strongly-ringed and spiraled tracheids, which are often collected 
into gland-like masses, the whole system conforming closely to the external 
form and following its morphological changes. 
Regarding the character of sunken stomata and their relation 
to hypoderma slight comment is necessary. The stomata are 
not sunken in the sense of having deep-lying guard cells; on 
the contrary, they are well upon the surface, as Schumann notes. 
Whatever vestibule there may be is of a different nature. With 
the increase in thickness of hypoderma an interval is produced 
3Recent problems in the anatomy, morphology, and biology of the Cactaceae. 
Bor. Gaz. 20: 130. 1895 
