A study in the Morphology of the Leaf. 247 
origin, as well as in its mature structure, characters which 
justify the conclusion that it is the result of a congenital 
coalescence of two pinnae. 
Turning now to the phyllopodium, that is, the whole leaf 
exclusive of the pinnae which constitute the lid, the pitcher 
itself is seen to originate first as an involution of the frontal or 
adaxial face of it, near to its apex, and the involution begins 
before the lid makes its appearance. It is brought about by 
the more active growth in an area surrounding a certain point 
than at the point itself. In this respect the pitcher of Nepenthes 
corresponds to that of Sarracenia or U tricularia, while over- 
growths of a similar order are not uncommon among other 
plants \ Dickson has compared the pitcher to the peculiar 
funnel-shaped, abnormal leaves of Croton , while Goebel has 
drawn the comparison between pitchers and orbicular leaves, 
and the pitcher may be regarded as an extreme instance of 
such slight hollowing of the upper surface as is to be found 
in such cases as Hydrocotyle , Tropaeolum , and N elumbium. 
Regarding then the pitcher as the result of involution of 
the upper surface of the phyllopodium at a point near its apex, 
it remains to consider the lower portion of the leaf. It has 
been pointed out in my paper above quoted that in very many 
cases, and especially in the Ferns, Cycads, and Dicotyledons, 
the phyllopodium is a winged structure throughout its length, 
and that those parts of it which may be distinguished when 
mature, depend for their differentiation upon the different 
mode of localisation of intercalary growth in them ; the parts 
usually recognised may be termed the hypopodium (or sheath), 
the mesopodium (or petiole), and the epipodium (or upper 
portion of the main axis of the leaf exclusive of the pinnae) ; 
but in my former paper it was expressly stated (p. 610) that 
4 such distinctions are only to be drawn where they are war- 
ranted by the exigencies of description/ In simple sessile 
leaves it is obviously impossible to distinguish those parts. 
1 The tubers of orchids, and of Phylloglossum , and inferior ovaries ; these, how- 
ever, involve the actual apex, while here it is not the apex which is arrested in its 
growth, but the point of arrest is at some little distance below it. 
S 
