134 The Botanical Gazette. [April 
and that the bristles, easily separable, rankle in flesh the of 
an enemy and impress its memory with the dangerous char- 
acter of that plant. But what purpose do the hooked ends of 
the spines so commonly found serve? Do they tear rather 
than prick an enemy? The spines show a great variety of ar- 
rangements; are these adaptive to the mode of attack of ene- 
mies? Very often there is a strong central spine pointing 
downwards, suggesting that it is to prevent a hoof from over- 
turning the plant. In other cases, as in Echinocactus hypa- 
canthus it points upward, suggesting that the plant may grow 
in hollows. In very many cases, as in Leuchtenbergia, spe- 
cies of Opuntia, and others, the spines become flat and papery 
and useless for protection. Of what use are they? In other 
cases they become flexible hairs. What useful end do they 
then serve? Is it possible that the strong protective spines 
occur upon the less extreme desert forms which live where 
for protection from loss of water is greater? In fact, do hairs 
and hair-like spines help materially top t hloss? Ormay 
it not be that they reflect and refract the too intense rays of the 
sun and mitigate its force for the green tissues??® Cana line be 
drawn between the hair-like spines, and the morphologically 
very different multicellular hairs? In Peireskia, some of the 
spines serve as hooks for climbing. What other uses and 
forms of them are found? It is plain that Geddes’ ebbing- 
vitality theory for the origin of spines does not apply in this 
vigorous group, nor is it likely that they are a direct result of 
dry climate as Lothelier would have us believe. Nowhere in 
nature is there a better place to test the dynamics of spine 
production than in this family. ; 
In the genera Mamillaria, Cereus, Rhipsalis, and Opuntia, 
species!? have been found which exude nectar in large clear 
drops from glands among the spines. These are particularly 
plain in O. arborescens. These glands have been proven to 
be spines more or less metamorphosed. Do they occur ! 
other genera? The exudation takes place only while new 
parts are being formed. What isits use? Are ants attracted 
eee 
1°Or as Coville (op. cit.) suggests, they may permit too great radiation oD the 
cold nights, i 
11A list of the known species is given in Flora, loc. cit. Nearly every spec!®> 
of Opuntia I have examined, eighteen in all, showed this nectar secretion. 
