446 fclNATlON. 



Some of the cases wherein a leaf seems to have a 

 double lamina may be alluded to here, tliough 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 ^o^. 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.) 



'It 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 Joum. Botan^jr,' vol. v, 1867, p. 323), and which 

 did not come under the writer's notice tiU after the sheet relating to 

 fission, p. 61, had been sent to pi-ess. Dr. Dickson thus speaks of this 

 abnormal leaf: " The petiole (unchanged) supported two lamina;, placed 

 back to back, and united by their midribs {i. e. not separated) to within 

 about an inch frotn their extremities, which were perfectly free from 

 each other. These lamina; stood vertically, their edges being directed 

 towai'ds 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 axU of this ab- 

 normal leaf were two axiUai-y 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 : \0 O/. The nature of the case may be even better seen 



by comparison with the normal arrangement, which would be q . 



while in those cases where the fission of the leaf occui's in the same 



{)lane as that of the primary lamina, as where a leaf splits into two 

 obes at the apex, with a midtib to each, the arrangement is as follows : 



X 

 V O / , the X in all cases representing the position of the axis, the 



() that of the axillary bud. and tho that of the liuninjr. 



