485 
the Monocotyledonous Leaf. 
expanded in the vertical plane to produce an isobilateral leaf similar to the 
equitant leaf of Tofieldia (Liliaceae, Fig. 20 A-c, p. 483) or Iris (Fig. 17 
A and B, p. 483). This comparison is facilitated by the fact that some Iris 
leaves, e. g. those of I. tenuifolia} have an oval transverse section, forming, 
as it were, a transition towards the Sisyrinchium type. That vertical 
flattening is in itself of no morphological significance, is demonstrated by 
the comparison of the cylindrical and flattened petioles occurring in different 
species, or even in different ontogenetic phases of the same species, within 
the genus Acacia. 
Certain species within the genus Iris , such as I. persica , 2 L., and 
I. orchioides , Carr., have leaves expanded in the horizontal plane with no 
flattened vertical region. These leaves are possibly equivalent to the leaf- 
bases alone in the case of the other Irises. 3 
The phyllode theory seems completely to explain the anatomy of the 
isobilateral equitant leaf; in this respect a comparison with the bundle 
system of various Acacia phyllodes proves illuminating. The comparison 
of an Iris leaf, with its two opposed series of bundles, to an Acacia 
phyllode has, indeed, been frequently made, 4 but no one seems to have 
hitherto taken the view that the similarity which they show yields the clue 
to the interpretation of the isobilateral equitant leaf. The flattening of 
a petiole in the vertical plane can scarcely take place without the production 
of two opposed series of bundles ; Fig. 2 B-D, p. 474, shows the actual 
anatomical effect of flattening and expansion in successive petioles of the 
seedling of Acacia neriifolia. The phyllode of this Acacia is closely com¬ 
parable in venation and anatomy with the leaf of Tritonia 5 (Iridaceae, Fig. 
15A-D, p. 483). In both Acacia neriifolia and Tritonia there is a special¬ 
ized region, which the writer proposes to call a 4 pseudo-midrib * ( p.m.r .), in 
which the two main laterals {m.l.) are concerned. Such a pseudo-midrib is 
a common character of the Tribe Gladioleae, 6 to which Tritonia and Gladiolits 
belong, while it occurs also in a slightly modified form in some species of 
Iris of the Section Tetragonae? Outside the Araceae it is found in Acorus 
(Araceae, Fig. 18 A and B, p. 483). In many other isobilateral equitant leaves 
no such distinct pseudo-midrib is evident, e. g. Tofieldia (Liliaceae, Fig. 20 
A-c, p. 483), Anigozanthos (Amaryllidaceae, Fig. 19 A and B, p. 483), and 
species of Iris not included in the Section Tetragonae , such as that 
1 Chodat, R., and Balicka-Iwanowska, G. (1S92). 
2 Balicka-Iwanowska, G. (1892-3). 
3 The present writer hopes to deal more fully with the morphology and anatomy of the leaves of 
the Iridaceae in a later paper, so all discussion of such forms as Crocus , Romulea , Cipura , &c., is 
omitted here. 
4 e. g. Bower, F. O. (1888). 
B The species examined and figured here was a garden hybrid, probably between Tritonia 
crocata , Ker-Gawl., and Montbretia pottsii, Baker. 
6 Chodat, R., and Balicka-Iwanowska, G. (1892). 
7 Balicka-Iwanowska, G. (1893-3). 
