214 The Botanical Gazette. [May, 
crowding together of the spine-clusters, suggesting that the 
stems shorten as roots sometimes do in the ground. Do they 
shorten, or is the crowding simply the result of the pressing 
together of the spine clusters as the curved ribs straighten 
out in growth? It is easy to see an advantage for the short- 
ening, as it would allow of a considerable development of 
new green tissue each year without an equivalent lengthening 
of the entire stem, and condensation is always an advantage 
to them. Moreover, in some stems, when cut across, the 
central cylinder soon shrinks down, showing that it is exert- 
ing a shortening pull upon the other tissues. I have seen this 
very marked in Mamillaria macrothele. 
In several genera, as Opuntia, Rhipsalis, Phyllocactus and 
Epiphyllum, flat shoots occur, differing somewhat morpholog- 
ically. These are usually jointed, and the joints commonly 
represent a year's growth. An important question arises as 
to how the flattening has been produced, and this has lately 
been investigated by Voéchting!* and Goebel.!7 The latter 
shows clearly that light exercises a direct (not intermediate 
through natural selection, etc.,) effect in causing the flat form 
and the growth of the tubercles in Opuntia, and the same 
probably holds good in other genera. Is it true in others and 
how widely? The cause of the jointing needs investigation. 
It isa common xerophilous character as seen in Euphorbias, 
Kleinia, etc. Is it primarily a growth condition or a charac- 
teristic acquired for biological advantage? In some cases at 
least it is utilized, for joints break off and are rolled away to 
start in a new place. 
It is important to search for other modes of protection 
in addition to spines. Do any Cactacee have a rank 
odor making them unpalatable to animals? Lewin has re- 
cently proven'® his earlier statement that Echinocactus 
(Anhalonium) Lewinii, a variety of A. Williamsii, contains a 
poisonous alkaloid, !® and he shows also that four or five others 
including a Cereus and Mamillaria are likewise provided. But 
it is not proven that these forms are thereby protected from 
animals. Are they? And if so, how do animals know that they 
are poisonous? Is it by smell or sight? They show no warn- 
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