206 



PRINCIPLES OP PLANT-TERATOLOGY. 



described, a series of transitions between a typically- 

 formed pitcher-leaf and a leaf possessing merely 

 laminar outgrowths on either side of the midrib. And 

 further, we are able to reach the striking conclusion, 

 from the consideration of the transitions which exist 

 between these laminar outgrowths and the attachment, 

 either by their midribs or petioles, of two entire 

 leaves, that these two phenomena are really one and 

 the same phenomenon; in other words, a leaf like that 

 of the phlox, Polygonum, the saxifrage, or even the 

 virescent stamen, represents merely a leaf in which 

 a second leaf is commencing to detach itself from, as 

 the case may be, the lower or the upper surface by the 

 process of what is above called ''facial fission."* This 

 conclusion receives support from such cases as that of 

 the elm-leaf (fig. 44), and the Syringa-le&i figured by 

 Celakovsky, in which, by a congenital infolding of 

 the apex or a portion of the margin, ventral laminae 

 become incompletely formed along the midrib or ~a 

 lateral vein from above downwards ; if this extended 

 to the leaf -base the result would be the formation of 

 two distinct leaves placed back to back.t 



In the plants mentioned above we see the congenital, 

 sporadic appearance, for what reason we know not, of 

 isolated stages in the process. It certainly would never 

 have been thought, before envisaging the phenomenon 

 from this view-point, that the ascending or pitcher-like 

 structure, when arising as a modification of an entire 

 leaf and not as a portion of the leaf, represents not, as 



* The infolding and union with the midrib of the ventral side of the 

 pitcher-leaf (i. e. a radially-constructed leaf), as in the saxifrage, may be 

 regarded as essentially the same phenomenon as that of " ring-fasciation " 

 in certain capitula and flowers. In both cases it must be held to be a stage 

 in the splitting of the simple organ into two. 



f A case of this sort would arise when the two median lamellae, or their 

 equivalents, on either side of the midrib, remained always distinct and 

 became parts respectively of the two daughter-leaves. These would thus be 

 separated off in a plane at right angles to that of the surface of the mother- 

 leaf. When, however, the lamellae become united together at either end, 

 this represents a stage in the separating off of two daughter-leaves in the 

 plane of the leaf-surface, and the two lamellae therefore become part of one 

 and the same daughter-leaf. Emargination or lateral forking of the mother- 

 leaf is, of course, a concomitant of the first process, but not of the second. 



