1895.] Present Problems in the Cactacee. 215 
ing colors, and if animals are not in some way allowed to dis- 
tinguish the poisonous from the harmless they would destroy 
the plants before discovering that they are poisonous, and the 
object of this mode of protection be defeated. Either the 
animals are warned, or else the presence of the alkaloid is in- 
cidental and not an ‘‘adaptation” to protection. Not only &. 
Williamsiit and closely related forms, but also the species of 
Anhalonium are without obvious means of protection, unless 
the hard cuticle of the latter serves to this end. The flattened 
and roughened and gray colored upper surface of A. fissura- 
Zum and the fact that they live partly sunken in the earth sug- 
gest that they may be protected by their resemblance to the 
ground on which they grow. Are they? The exudation of 
nectar already mentioned is perhaps protective, and the ar- 
rangement of crystals in a sort of armor just under the epi- 
dermis of Opuntia arborescens (to be described below) is prob- 
ably protective against snails. Are any others to be found 
in the family? A red color occurs on the under surface of 
Peireskia leaves, on young shoots of Cereus, etc., and on 
nearly all seedlings. Is this, as Stahl would suggest, associ- 
ated with the presence of tannin or other distasteful substance, 
and hence of the nature of a warning color? Or is it in the 
Seedlings a light-screen? The probability of some protection 
against too great light and heat has already been mentioned, 
and possibly the hairs, or even the hypoderma may assist in 
this,?° but nothing positive is known about it. The investi- 
gation of the minimum, optimum, and maximum temperature 
points for some desert Cactacee would give results of great 
interest. There is reason to think that electrical currents are 
to be detected on the deserts.*!_ Do plants show any rela- 
tions to them? 
The entire relation of form-conditions to climate requires 
more careful study.?2 Is condensation proportional to dry- 
rete eramnmasnosdiouaany 
7° See McFarlane’s suggestion, Bot. Centr. 51: 184. 1895. 
*1 National Geographical Mag. 4; 171. 1893. : oe 
?? As a basis for such study, the forms se-m to fall into divisions about as 
follows. Goebel has traced the subject for the genus Opuntia. i 
a. The branching, shrubby type, little removed from the typical condition: the 
woody Peireskias. 
ribbed: Cereus, Pilocereus, etc. —_ 
upright } tubercle fleshy Peireskias, Cylin- 
b. The unjointed col race yee 
. Cotwmn®) creeping or deflexed: creeping Cereus and Echb- 
inocereus. 
