36 BOTANICAL GAZETTE [JULY 
the doubtful species in each genus. The single genus Cereus 
contains a multiplicity of forms, the structures of which must be 
very diverse. How are these related, and what are their several 
connections with the other genera? Does the gradual change 
in internal structure agree with the external and strengthen the 
validity of the various systematic groups as now recognized ? 
Schumann? has pointed out that the original ancestor of the Cac- 
taceae was probably something like Peireskia. Ganong? has 
followed up and confirmed the relation between this genus and 
Opuntia, chiefly on the basis of external characters. What are 
the steps by which the corresponding internal structure has 
changed? Schumann’ has hinted rather strongly at the con- 
necting links between Opuntia and Cereus; anatomical studies 
may give more definite information upon this point. 
This paper seeks to impress the importance of careful struc- 
tural study to help out the ordinary work on external characters, 
to point out the general trend of evolution exhibited, and to 
interpret certain anatomical conditions upon a physiological basis. 
As regards environmental adaptation, next to diminution of 
surface and other modifications to lessen transpiration, the stor- 
age of water is most important. The turgor exhibited in many 
of the more succulent forms is surprising. Especially is this the 
case in the younger portions of the arborescent species, and 
throughout the interior of those which are low and cespitose, 
where for a long time turgor alone is sufficient to sustain all the 
weight placed upon it—-in other words, to preserve the rigidity 
of the plant. Hence there is no necessity for a strong vascular 
system ; in fact, its room is more valuable than its presence, and 
we find in general that the vascular system is of late develop- 
ment, and in low forms almost never reaches a high state of 
perfection. The component elements also are oftenest of a 
kind generally considered primitive, showing an apparent retro- 
gression, but one which in many cases is complicated by the 
1 Gesammtbeschreibung der Kakteen. 1897. 
?Beitrage zur Kenntniss der Morphologie und Biologie der Kakteen. Fiora 
79: 49-86. 1894. 
