476 Agnes Arber .— The Phyllode Theory of 
if the ancestral petiole, from which the phyllode was derived, happened to 
have an open arc of bundles; inverted strands are also lacking in the 
phyllodes of Oxalis bupleurifolia , whose petiolar nature is uncontested. 
Again, though some Monocotyledonous leaves are equivalent, on the 
phyllode theory, to leaf-bases plus petioles , it is probable that others are 
reduced to leaf-bases alone , and in these only normally orientated bundles 
would naturally be found (cf. bud-scales of Ribes nigrum). The leaves of 
certain species of Iris, for instance, are best interpreted as corresponding to 
the leaf-bases alone of other members of the genus. 1 
(iii) Inverted bundles in Monocotyledonous leaves. 
To avoid obscurity, it may be well at this point to anticipate the 
succeeding sections of this paper so far as to state that the result of 
a general examination of the leaves of Monocotyledons is to reveal the 
frequent occurrence of phyllodic anatomy, especially in the more primitive 
Families. In order to leave no doubt as to what is here intended by the 
term ‘phyllodic anatomy’, it may be pointed out that all the cases repre¬ 
sented in Figs. 6 to 14, p. 479, show the inverted bundles (i.b.) which the 
present writer regards as indicative of phyllodic structure. Figs. 9 and 10 
are instances in which relatively little modification of the original petiolar 
anatomy has apparently occurred. 
(iv) Midrib anatomy. 
It may be objected, with some force, that the type of anatomical 
structure here called phyllodic might equally well be taken to indicate that 
the organ showing it is derived from the midrib region of a Dicotyledonous 
leaf, since many midribs (and some main laterals) closely resemble petioles 
internally. 
On the other hand, there seems to be no positive evidence for the view 
that the typical Monocotyledonous leaf represents the midrib of an 
ancestral lamina. The simplicity of form in such a leaf; the uniformity of 
structure from the top of the leaf-base to the leaf-apex ; the lack of any 
indication of external lateral appendages; the absence of any vestigial 
internal trace of pinnate or palmate venation ;—all these are points suggest¬ 
ing derivation from a leaf-base and petiole alone, rather than a more com¬ 
plex origin. 
But since the midribs and main laterals of Dicotyledonous leaves may 
resemble petioles in structure and may show inverted bundles on the adaxial 
side, no Monocotyledonous genus is included in the list of phyllodic cases 
(pp. 478-81) on the strength of its showing inverted bundles in the main ribs 
alone. It is only the occurrence of such bundles outside these ribs which 
1 See p. 485. 
