231 
ter to the margin of the leaves of the same species.” With regard to 
the Firs, the anatomical points of most importance are; the resin 
ducts of which there are always two in the Abies leaf, readily seen 
in a horizontal section; the presence or absence of stomata on the 
upper surface; and the presence, distribution and relative size of 
the hypoderm cells—robust longitudinal cells, with thick walls and a 
very slender cavity, almost always present on the edges and keel of 
the leaf and often forming a stratum on the upper side. ‘These cells’ 
seem to form the most certain distinction between A. Fraseri, Pursh, 
of the high mountains of North Carolina, our A. balsamea, Mar- 
shal, and A. subalpina, Engelmann; the first having an almost 
uninterrupted stratum of hypodermic cells on the upper side of the 
leaf; the second scarcely any ; and the third, which is the western 
representative of da/samea, more abundant cells above than under it, 
and crowded on edges and keel. The next four species have the resin 
ducts close to the epidermis of the lower side, towards the edges. 
A. grandis, Douglas, and A. concolor, Engelmann have the hypo- 
derm cells in interrupted strata under the upper surface of the leaf; 
A. religiosa, H. B. K., and A. bracteata, Don, in a continuous layer. 
A. grandis has no stomata above, A. concolor has. A. bracteata has 
glabrous scales, and may thus be distinguished from the Mexican A. 
religosa and all other firs. The 8th and goth species have the resin 
ducts, like the last four, close to the epidermis of the lower side, and 
in fertile branches equidistant from the edges and keel, but it is 
questioned whether 8, A. nobilis, Douglas, and 9. A. magnifca, Mur- 
ray, are really distinct; Dr. Engelmann, inclines to think they are; 
Drs. Hooker and Gray, that they are not. In giving this diagnosis 
we have omitted all the other distinctions of more service in deter- 
mining the trees when met with, as we wished to give an idea of 
these crucial anatomical points in the case of doubtful specimens, 
and we have not perhaps done justice even to these. After all it is 
very difficult to tie nature down to logical exactness. We see in 
Nature, Ap. 18th, that Mr. J. Gorham, in a paper read before the 
Linnean Society undertakes to detect the genus of an umbelliferous 
plant from an examination of the venation of the merest fragment. 
We wish we. had room to extract some more graphic parts of the 
description of these noble trees. 
§ 236. Opuntia Ficus-Indica, DC —I have received from Dr. C. 
H. F. Peters, 6f the Hamilton College Observatory, the following 
note, commenting on my note in the BULLETIN lately. He was 
“reminded of the hedges in Sicily and of the impenetrable cactus - 
fields on the lavas of Mt. Etna. It is the plant first of all used to 
break down the lava beds. A handful of soil is put down, and a 
cactus joint (or leaf) thrown upon it. That soon takes root, and the 
rock weathers very fast. So that, after a few years, as the second 
plant for aiding in making vegetable soil, fig trees are set out. I 
liked the fruit of the catcus very much.” 
You may remember that Dr. Peters made the first thorough sur- 
vey of Mt. Etna, before 1840, at the expense of some German duchy. 
His survey is quoted in Humboldt’s Cosmos. ip, 
§ 237. Rediscovery of a lost Sphaeria.—On returning from a 
