certain Monocotyledonous Leaves. 347 
IV. Comparison between the Ontogeny of the Blade in 
Monocotyledonous and Dicotyledonous Leaves. 
In the earlier part of this paper (pp. 330-43 and Figs. 1-2 1) I have 
reviewed the mode of development of a number of Monocotyledonous leaves 
belonging to that type — somewhat exceptional within this Class — in which 
there is a definite distinction between stalk and blade. In undertaking this 
study I have had in mind the idea that such blades are not true laminae, but 
represent modifications of the distal part of the petiole ; and I thought that 
a comparison of the development of these ‘ pseudo-laminae ’ with that of the 
laminae of Dicotyledonous leaves might be a help in estimating what 
degree of validity could be claimed for this idea. It seemed to me that 
the severest test of the pseudo-lamina conception would be to compare the 
development of the Monocotyledonous leaves in question with that of 
Dicotyledons whose blades resemble them in type, so that a number of 
differences of an obvious kind would be at once eliminated. In many 
of those Monocotyledonous leaves which have a differentiated blade, this 
blade is either ovate in form (often with a tendency to the production of 
a cordate or auricled base), or else it is plicated in a fan-like manner. The 
Dicotyledons whose leaf-development has just been described (pp. 335 and 
346 and Figs. 22-9) were chosen because their blades conformed to one of 
these types ; in a few cases they were also selected for some special reason — 
Aristolochia because of Monocotyledon-like features in its anatomy, and 
Polygonum because of the resemblance in construction between the leaf of 
this genus, with its ochrea, and that of certain Potamogetons. The result 
of my observations — stated in the most general terms — is that the lamina 
of those Dicotyledons which I have examined arises as wing-like out- 
growths from the sides of the petiole, while in some cases the petiole itself 
may be regarded as undergoing lateral expansion as well as winging. In 
certain Monocotyledons the ‘lamina’ arises in exactly the same way. 
As regards the general scheme of their development, such leaves as those 
of Potamogeton (Fig. 1, p. 331), Hydrocharis (Fig. 4, p. 331), Calla (Fig. 8, 
p. 336), Rhipogonum (Fig. 16, p. 338), and Tamus (Fig. 18, p. 341) are scarcely 
distinguishable from those of Dicotyledons. As far as these leaves are 
concerned, it must be admitted that the developmental evidence affords no 
active support to the pseudo-lamina theory ; such a blade, for instance, as 
that of Tamus (Fig. 18, p. 341) recalls in its development that of Aristolochia 
(Fig. 22, p. 344), while those of Potamogeton (Fig. 1, p. 331) and Aponogeton 
(Fig. 3, p. 331) resemble that of Populus (Fig. 23, p. 344). Such negative 
evidence does not, however, disprove the pseudo-lamina theory, for a blade 
derived from a petiole in the simplest possible way — namely, by flattening, 
expansion, and winging — could scarcely fail to present an exact simulacrum 
of a true lamina. 
