1895.] Present Problems in the Cactacee. 135 
by it which protect the young growing parts against some 
creeping tissue-eating enemy! 
The spines are believed to originate always strictly dorsi- 
ventrally from the vegetative point, thus agreeing with one 
method of production of leaves. This has been proven only 
for Opuntia arborescens. It is desirable to investigate this 
point in other species and genera. In Peireskia however the 
spine-production is nearly radial; in Opuntia it becomes dor- 
siventral, the large spines being produced upon the outer or 
leaf side of the axillary vegetative point, and the bristles upon 
the inner or stem side. The dorsiventrality becomes still 
more marked in Cereus, etc., where nothing, or only some mul- 
ticellular hairs, is produced upon the inner side; still more so 
in those species of Echinocactus where the point splits into 
two parts, and yet more in Mamillaria where it splits into 
two parts, which separate entirely. Is there any case in which 
the outer part of the point in Mamillaria produces its spines 
radially, or are they all laid down dorsiventrally? at a 
strong central spine closes the growth, as sometimes stated, 
is altogether improbable. 
In Petreskia aculeata a very early splitting’? of the axillary 
point has been found, one part remaining upon the leaf base, 
the other being carried up by the young stem in its growth, 
an exact reversal of the condition in Mamillaria where, after 
the splitting, one part remains in the axil and the other is 
Carried out by the growing tubercle. Is this condition found 
in other species of Peireskia? The lower point normally pro- 
duces the new branch, but when it is destroyed, the upper 
One does so. From the descriptions it seems as if the flowers 
are produced from the upper. Is this true? In P. aculeata, 
it is possible that the axillary bud does not at first stand upon 
the leaf base as in other genera, but in the axil in contact 
with both leaf and stem. Is this true? It is difficult to 
point could be carried up by growth of the stem. The trans- 
ition from Peireskia to Opuntia is perfectly gradual and there 
can be no doubt that Peireskia is the nearest of all living 
Cactacez to the original stem-form. In P.aculeata and P.bleo 
a single large leaf often appears in the center of the spine mass. 
It seems impossible that this is formed in any other way than 
Pig splitting of the point so often referred to, is, I believe, unique in this 
by the It is nota branching nora bifurcation, but a division into two parts 
going over of some of its central meristem into permanent tissue. 
