44.6 ENATION. 
Some of the cases wherein a leaf seems to have a 
double lamina may be alluded to here, though possibly 
they would more properly be referred to fission. The 
appearance presented is as if four wings projected from 
the midrib, so that a cross section would be nearly in the 
~~ . . 
form of © . In an orange leaf presenting this ap- 
pearance the lower surface of one lamina was, as usual, 
dull in colour, while the upper surface of the subjacent 
lamina was likewise dull; hence the impression might 
arise that this was an instance of the adhesion of two 
leaves back to back, but the petioles were not twisted, 
as they must have been had two leaves thus been 
united, and neither in the petiole nor in the midrib 
was there the slightest indication of fusion, the vascular 
bundles being arranged in a circular manner, not in a 
horseshoe-like arrangement, as would have been the 
case had adhesion taken place.' (See p. 33.) 
' Tt is desirable in this place to allude to a singular case of fissiparous 
division of a leaf of Prunus Laurocerasus described by Prof. Alexander 
Dickson (‘Seemann’s Journ. Botany,’ vol. v, 1867, p. 323), and which 
did not come under the writer’s notice till after the sheet relating to 
fission, p. 61, had been sent to press. Dr. Dickson thus speaks of this 
abnormal leaf :—‘* The petiole (unchanged) supported two laminz, placed 
back to back, and united by their midribs (7. e. not separated) to within 
about an inch from their extremities, which were perfectly free from 
each other. These lamine stood vertically, their edges being directed 
towards and away from the axis; and as they were placed back to back, 
the shining surfaces, corresponding in structure to the normal upper 
leaf-surface, were directed laterally outwards. In the axil of this ab- 
normal leaf were two axillary buds. The existence of two leaf-apices 
and two axillary buds shows that this was not due to an accidental 
exuberance of development, but to fissiparous division, which, had it 
been complete, would have resulted in the replacement of a single leaf 
by two leaves. The arrangement in Prof. Dickson’s leaf may be thus 
x 
represented : \O O/. The nature of the case may be even better seen 
5 C ‘ x 
by comparison with the normal arrangement, which would be o . 
——" 
while in those cases where the fission of the leaf occurs in the same 
plane as that of the primary lamina, as where a leaf splits into two 
lobes at the apex, with a midrib to each, the arrangement is as follows: 
x 
Say , the x in all cases representing the position of the axis, the 
O that of the axillary bud, and the —— that of the lamina. 
