/*' 
79 
carbonic anhydride gas, causes unequal growth along their mutual 
limits and their outlines become broken and assume irregular shapes; 
segments growing faster than others produce the diversified serra- 
tions, elongations, partings and fimbriations which characterize the 
leaves of different plants, all this however taking place in subordina- 
tion to the ancestral peculiarities of the plant by which a general 
ground form or architectural type is preserved. Looking at the 
leaves of /^, toxicodendron^ we find them arranged in pedunculated, 
terminal sets of three, one distal and two lateral leaflets. The distal 
or central leaflet normally, and putting aside the divergent shapes 
found in this plant, is rhombic-ovate, equilateral, and symmetrically 
toothed on each side, the two side and proximal leaflets aresubrhom- 
bic-ovate, inequilateral, and toothed conspicuously on only one side 
of the free margin. In other w^ords, along the edges of the two 
lateral leaves where they come in conflict, especially with the edges 
of the terminal leaf, a restriction of growth takes place, and the usual 
lobation seen in the end leaf, and which may be considered typical, 
is suppressed* and on the margin, which is relieved from any compe- 
tition, this lobation appears. Does- not this contradict Mr. Allen's 
assumption ? That there is a struggle for nourishment and that the 
terminal leaf, in the direct line of the sap's flow, is favorably placed, 
is seen by the slight growth of the halves of the side-leaflets facing 
the former, and the natural character of the halves away from it. 
But the tendency is to obliterate serration at the parts mentioned, 
and might, we should suppose, under persistent repetition, form an 
entire edge. A very similar condition of things, with some mterest- 
ing details, is seen in the leaf-clusters of Negundo aceroides. An 
interesting change of form in the leaves of the poison ivy takes place 
when they have l)een stung by a species of gall-insect (?) at the apex 
of a leaf. In the terminal leaf it produces an arrest of growth, a 
deep cleft, and lateral enlargement, which destroys the notched out- 
line, enveloping the lobes in a rounded full blade. 
Of course it is not difficult to explain the abortive character of 
the one side of the leaflets mentioned above. It is due to the pre- 
dominant shade caused by the expanded sides of the terminal leaflet, 
which owes its vigour to its favorable position. The conditions pre- 
sented here are apparently what Mr. Allen would wish, two neighbor- 
ing leaves contending for the molecules of carbonic anhydride and 
evidently unequally matched, as appears from the result ; but the re- 
sult does not seem desirable for his theory, as the characteristic 
toothing is suppressed, not exaggerated in the weaker, and this 
notching, which he seems to regard as due to impeded growth, is 
best shown in the stronger competitor. 
L. P. Gratacap. 
r 
r 
The Fertilization of Opuntia.— For the purpose of adding bril- 
liancy of color to the window-garden in front of my house, I planted 
in two separate boxes a large number of plants of O^nntia vulgaris, 
which I obtained from near Hartsdale, Westchester County, N. Y., 
' -- ■- ■ ■ 
*Mr. A. Holliclvgave me the name of this tree, which I had previously ob- 
^ved in connection with this subject. 
