310 Arber . — The Leaf Structure of the Iridaceae , 
The interest of the winged axis, from the standpoint of the phyllode 
theory, lies in the fact that, just as the main features of the isobilateral 
equitant leaf of the Irids may be compared with those of Acacia phyllodes, 
so even the minor peculiarity of the winged axis finds a parallel among the 
phyllodic Acacias. Acacia alata , R. Br. (Figs. 24 A and B), has two opposite 
rows of reduced leaves, whose downward continuation produces a winged 
axis bearing a general resemblance to that of Sisyrinchium, &c., though the 
occurrence of the wing on the two sides is here due to the fact that each of 
the two alternating phyllode bases is decurrent through two internodes. 
Transverse sections show that the parallelism is not confined to mere 
externals (cf. Figs. 24 B and 25 E) ; in both cases there is a central axial 
region (ax .) — radial in structure and surrounded by a sheath of fibrous 
tissue — entirely distinct from the associated lateral wings (/.). 
3. Non-ensiform Leaves in certain Iridoideae. 
(i) ‘ Radial ’ leaves in Iris and other Iridoideae , and their relation to 
Acacia phyllodes. 
Within the genus Acacia we find every transition between phyllodes 
which are completely flattened in the vertical plane (Figs. 14, p. 304, and 
41 and 42, p. 318) 1 and others which are cylindrical and terete (Fig. 28, 
p. 31 1). 2 In various Iridoideae we meet with the same passage from equitant 
leaves to others that are more nearly radial. The leaves of most Irises of 
the Apogon Section are characteristically ensiform (e. g. Iris Pseudacorus , 
Figs. 1 A and B, and I. spuria , Fig. 8), but the genus also includes ‘ radial ’ 
leaves. Iris tuberosa , L. (Hermodactylus tuberosus , Mill.), is a plant in 
which the ensiform Iris leaf has been modified into a remarkable tetragonal 
form. Figs. 29 A-c, drawn from a microtome series, show that at the 
extreme base there is a closed sheath (Fig. 29 a) which opens out a little 
higher up (Fig. 29 B) and gradually passes into a solid limb, of diamond- 
shaped section (Fig. 29 C). The median strand (rn.b.) and main laterals 
(m.l.) occupy three of the four angles, and the ventral margin ( v.r .) can be 
distinguished by the fact that it has no bundle with the xylem pointing 
directly inwards. Higher up, in the mature part of the leaf (Fig. 29 D), 
we find that each of the four ribs has grown out into a slight keel, supplied 
as a rule by two small bundles (b. x and b. 2 ) which appear to arise as 
branches from the main bundles. Close to the apex the fibrous strands, 
which occupy the angles of the leaf, become increasingly conspicuous 
(Fig. 29 E). 
The bulbous Irises of the Tetragonae Section (/. reticulata , Bieb., 
I. Histrio , Reichb., /. Vartani , Foster, and I. Bakeriana , Foster) have 
1 See also Arber, A. (1918), Fig. 2 d, p. 474, and Fig. 21, p. 483. 
2 Ibid., Fig. 1 b, p. 47 a. 
