475 
the Monocotyledonons Leaf\ 
donous petioles—both those of normal form and those known as c phyllodes ’, 
which are expanded and lamina-like ; we must also touch upon the anatomy 
of those scale-leaves which are equivalent to leaf-bases only. 
The structure of petioles 1 varies too much to lend itself readily to 
generalization. Disregarding a large number of minor complications, we 
may say, however, that most petioles, as seen in transverse section, are 
characterized by a more or less open arc or a complete circle of bundles; 
the result is that, although the majority of the bundles are usually orientated 
as in the lamina, certain of them, on the ventral or adaxial side, may come to 
be inversely orientated. The position, hence, is that inverted bundles are 
absent in typical Dicotyledonous laminae (excluding the principal ribs) and 
are sometimes present and sometimes absent in petioles ; their presence is thus 
a distinctively petiolar character, but their absence is a character common to 
laminae and to some petioles. 
The best-known examples of petiolar phyllodes are those found in 
a number of species of Acacia (Figs, i A and B and 2 A-D, p. 474, and 
Fig. 21, p. 483) and Oxalis (Fig. 3 A and B, p. 474). 
In the case of Acacia , the phyllodes, in the majority of species, are 
expanded in the vertical plane, but they may also be more or less radial in 
structure (Fig. 1 A and B, p. 474 ), while in one species they are described as 
horizontally expanded. 2 The chief anatomical feature in which they diverge 
from true laminae is in the occurrence of two series of bundles of opposed 
orientation (Fig. 2 D, p. 474, and Fig. 21, p.483). Owing to the fact that in 
some species the earliest leaves of the seedlings have petioles that are 
scarcely flattened, we are able to trace the changes in the anatomy of the 
petiole as it becomes phyllodic (Fig. 2 B-D, p. 474). 
The phyllodes of Oxalis bnpleurifolia , A. St. Hil., which are expanded 
in the horizontal plane, do not show the two series of bundles characteristic 
of Acacia , but have one series of normal bundles, the marginal ones being 
horizontally placed (Fig. 3 B, p. 474). As an example of a bud-scale we 
may take that of Ribes nigrum , L. In this plant the normal petiole has an 
arc of xylem and phloem, almost meeting on the upper side to form a com¬ 
plete circle in transverse section (Fig. 5 b, p. 474). But in the bud-scales, 
which are obviously of leaf-base nature, the vascular supply is reduced to 
separate parallel bundles orientated as in an ordinary lamina (Fig. 5 c, 
P* 474 )* 
The study of normal and phyllodic Dicotyledonous petioles and scale- 
leaves thus leads us to two conclusions: firstly, that the presence , in 
a Monocotyledonous leaf, of additional adaxial bundles with inverted 
orientation may well be interpreted as affording support to the phyllode 
theory ; secondly, that the absence of such inverted bundles would in no 
way invalidate the theory. For their presence could scarcely be expected 
1 Petit, L. (1887 an d 1889). 2 Hochreutiner, G. (1896). 
