A study in the Morphology of the Leaf. 249 
lamina are based 1 * * . These wings are early recognisable in the 
developing leaf, and are usually present on its adaxial face ; 
they are the seat of important subsequent growths ; their 
most prominent mode of development is the formation of 
the lateral flaps of the simple lamina ; or upon each of 
them may arise in longitudinal series the pinnae, which may 
remain rudimentary, and show congenital coalescence (as in 
leaves with serrate margins), or the pinnae may develop 
separately from one another (as in the truly pinnate leaf). 
The prevalence of these wings from the Ferns upwards, and 
the influence of their existence upon the ultimate confor- 
mation of leaves, makes it appear to me to be important to 
recognise their presence more fully than is usual ; the recog- 
nition of the phyllopodium, as usually a winged structure, 
seems to me to lead to a better insight into the real nature of 
the leaf than the usual custom of drawing at once the trans- 
verse limitations of sheath, petiole, and lamina. 
The phyllopodium, thus recognised, is susceptible of various 
development throughout its length, according to the varying 
balance of transverse and longitudinal growth. Where the 
transverse growth is relatively great, and the longitudinal less, 
the result is such a development as that which we usually 
term a lamina. Where the longitudinal growth is in the ascend- 
ant the result is what we call a petiole. Beyond the fact that 
these differences of localisation obtain respectively in those 
two parts, and that the petiole is intercalated at a relatively 
late period, there seems to me to be no essential or recondite 
1 This expression of opinion is diametrically opposed to that of K. Reiche 
(Ber. d. D. Bot. Ges., 1888, p. 328), who remarks as the result of the observation 
of winged stems and decurrent leaves of certain Phanerogams as follows : — 4 Dass die 
Fliigel an pflanzlichen Organen aus geringen, morphologische Charactere darstel- 
lenden Bildungen hervorgegangen seien.’ This conclusion is based on a narrow 
comparison. The author does not refer to any vascular Cryptogams or Cycads, and 
generalises from observations on a few stems to ‘plant-organs’ generally. Now 
the morphological importance of characters depends upon their prevalence through 
large groups, and the prevalence of a more or less clearly winged character in the 
leaf from the Ferns upwards is accordingly a character of morphological import- 
ance; certainly its importance in the leaf is not to be set aside because it is not 
prevalent or constant in the stem. 
