488 
Agnes Arber .— The Phyllode Theory of 
Instead of regarding a peripheral series of bundles as an adaptation 
developed in response to the succulent and xerophilous habit, the present 
writer looks upon the existence of such bundles as an indication that the 
leaf in question is morphologically equivalent to a petiole, or to a petiole 
and midrib ; but, since peripheral bundles happen to be adapted to the 
requirements of a succulent leaf, they may well have been one of the factors 
concerned in rendering a xerophilous habit possible. On this view, the 
‘ radial 5 leaf, whether it belongs to a Dicotyledon or Monocotyledon, is 
regarded as owing its form and structure primarily to its morphological 
nature, the adaptational use of its structural peculiarities being entirely 
secondary. 
It may be added that it seems scarcely possible to suppose that the 
inverted series of bundles in the relatively thin leaf of the Daffodil (Fig. 14, 
p. 479), for instance, can be of ‘survival value or can represent a special 
adaptation to the mode of life, while such bundles are absent in the leaf 
of the Bluebell. The general structure and mode of life of the two 
plants is closely similar, and the Daffodil does not show any obvious 
superiority in the struggle for existence. But this point must not be 
laboured, since it involves the whole vexed question of the meaning of 
adaptation. 
6 . The anatomical evidence for Henslow’s corollary to the phyllode 
theory, with special reference to the Pontederiaceae. 
Professor Henslow’s corollary to de Candolle’s phyllode theory has 
already been outlined (p. 470)- He bases his view entirely on external 
morphology and the macroscopic characters of the venation, but the present 
writer wishes to draw attention to some cases in which his theory appears 
to receive definite support from the anatomical structure of the leaves in 
question. 
The Pontederiaceae will be considered in some detail in this connexion, 
since the peculiarities of their leaf anatomy seem hitherto to have escaped the 
attention of botanists. The leaves of this Family generally have a sheathing 
leaf-base, a petiole which is sometimes much swollen, and a ‘lamina 5 . 
In external appearance and venation the leaves of Pontederia (Fig. 23, p. 489) 
and Eichhornia (Fig. 24, p. 489) distinctly suggest that the ‘ laminae ’ are 
produced by expansion of the apical regions of the petioles, and that they 
are thus ‘ pseudo-laminae ’ and not equivalent to the blades of Dicotyle¬ 
donous leaves. The anatomy confirms this idea in a striking fashion. 
Fig. 28, p. 490, shows the transverse section of a petiole of Pontederia 
cor data, L., with inverted bundles towards the upper side. When the 
‘ lamina ’ is cut transversely, its structure is found to be exactly such as 
might have been anticipated on the theory that it is produced by extreme 
flattening and expansion of the petiole in the horizontal plane (Fig. 36, 
