THE LEAF. 



207 



we usually suppose, a single leaf whose margins have 

 become congenitally united, but in reality two leaves. 

 This is obvious enough in the terminal pitchers of the 

 Buddleia and Marrubium, where the normal leaves 

 occur in pairs on the stem below ; it is less so in the 

 case of the pitchers of Saxifraga and Pelargonium 

 which have alternate leaves ; but the transitional 

 phenomena observed in the former, and the presence of 

 a marked midrib on the side opposed to that of the 

 normal midrib in the latter, seem to justify this con- 

 clusion. The terminal leaf of Buddleia with a basal 

 pocket and the pitcher-leaf of Pelargonium seem to be 

 precisely similar structures. 



The formation of ascidia and of ventral laminae is, 

 apart from the interpretation of them above given, an 

 absolutely inexplicable and bizarre phenomenon. That 

 an ordinary bifacial, dorsiventral leaf should suddenly 

 appear as a pitcher, or suddenly produce strange 

 laminse on its surface, is surely incongruous with all 

 we know of the characters of a leaf. This incongruity 

 and inexplicability vanish, however, if the above 

 interpretation of the phenomenon be admitted. 



These phenomena have been attributed as a whole 

 to fission, more perhaps for the sake of convenience of 

 description than anything else. But although all are 

 certainly of the same morphological nature, it would 

 be obviously a mistake to attribute them all to fission. 

 Just as in the case of forked leaves, so also here : some 

 must be due to fission, others to fusion (union). In 

 some cases, where we have only detached leaves, with- 

 out the possibility of observing the leaf-arrangement 

 on the shoot which bore them, as in the case of the 

 mango-leaf, we cannot determine whether the abnormal 

 leaf is due to fission or to union of two leaves. In the 

 case of the saxifrage-leaves, in the absence of any 

 evidence of fusion, we must attribute fission as the 

 cause; the same applies to the Phlox-] eaves, and 

 possibly to the Pelargonium-leaf. 



In the case of the double leaf of the Buddleia, in- 



