the Embryos and Seedlings of the Cactaceae. 44 1 
hand Cylindropuntiae which grow in the moister climates, 
and on the other, Platopuntiae which grow in the extreme 
deserts, but at present I have not material for this comparison. 
In any case there can be no doubt that the relatively larger 
and flatter cotyledons of the Platopuntiae do not indicate 
that they are more nearly related to Pereskia than are the 
Cylindropuntiae, for all evidence is against such a possi- 
bility, but they are the result of a re-enlargement allowed by 
the moister climate in which they grow. Another explana- 
tion for the cotyledons of O. basilaris is that this form has 
come off from the Cylindropuntiae entirely independently of 
the other Platopuntiae, and has retained the primitive leaves. 
The differences between the two divisions of Opuntia are 
shown clearly also in the cellular anatomy, but I shall here 
cite but one phase of this. The Cylindropuntiae have a six- 
bundled central cylinder, as shown in Fig. 2 d, while the 
Platopuntiae have a two- or four-bundled cylinder arranged 
as shown in Fig. 5 d. 
The cotyledons, normally two, are sometimes one, often 
three ; and sometimes, as in O. vulgaris , elsewhere described, 
which is polyembryonic, there may be more, with some 
imperfect. In Platopuntiae they are unequal in size, and it 
is easy to see that the longer is on the convex side in the 
seed, and the shorter on the concave. No doubt this in- 
equality of the cotyledons is not in the least to be traced 
to adaptation, but is a simple result of position in the seed, 
which gives less room to one than to the other, hence allowing 
it to make fewer cells ; and as germination is little more than 
the swelling of cells already laid down, the cotyledons must be 
unequal. Many Platopuntiae have the cotyledons placed 
incumbently in the seed, while some Cylindropuntiae have 
them accumbently, and one at first attributes the flat coty- 
ledons of the former and the nearly half-cylindrical form of 
the latter to this position : but an inspection of the many fine 
figures of embryos and seeds given by Engel mann disproves 
such a connexion. 
The cotyledons have axillary buds, which I have made to 
H h 2 
