312 
Arber . — The Leaf Structure of the Iridaceae , 
is ventral, while the unequal surfaces, j and /, are lateral. Towards 
the apex the leaf becomes horny ; sections show that this horniness is due 
to a thickening of the epidermal walls. 
Sisyrinchium is another genus in which both ensiform and terete leaves 
occur. Fig. 27 shows the oval ribbed limb of S . junceum , E. Mey. ; 1 it may 
be compared with the phyllode of Acacia teretifolia , Benth. (Fig. 28). The 
resemblance is remarkably exact, the vascular strands showing a close 
approximation in number and distribution. 
In the genera Trimezia and Bobartia the occurrence of cylindrical 
j unciform leaves has also been described. 2 
(ii) Transitional and bifacial leaf -types in Iris and Moraea. 
The Irises to which I have hitherto referred are characterized by the 
unimportance of the leaf-sheath as compared with the limb — at least in the 
mature leaves — but in the bulbous Irises of the Section Xiphium , the sheath 
takes a more prominent place. In Iris xiphioides^ Ehr., the English Iris, 
there is a cylindrical limb (petf usually of some length (Fig. 31 A), 
but which may be reduced to a mere trace (Fig. 31 b). I regard 
such a leaf as a leaf-base (sh) crowned by a petiole, which may be either 
well developed or vestigial. The evidence on which such a view can be 
based is set forth in a previous paper, 3 in which I have shown that the 
leaves of certain species of Ttdipa , Hyacinthus , &c., terminate in a cylindrical 
apex, which by analogy with the transitional forms between the mature 
leaf and the bud-scale in the Dicotyledon Fatsia , I interpret as petiolar. 
In Iris xiphioides. we have a leaf of similar morphology, which, however, 
reveals its nature much more obviously. The apical region of I. xiphioides 
is approximately cylindrical, and its anatomy is so nearly radial that, if 
transverse sections be taken through it above the sheath, it is impossible 
to be certain about the orientation from internal evidence. In Fig. 31 D, 
however, the section passes through the extreme top of the junction of 
sheath and limb, where the last trace of the sheath cavity,^., indicates which 
side is adaxial. 
The related species, Iris Xiphium> L., the Spanish Iris, is significant 
because the apical limb is but little modified from the ensiform type 
(Fig. 32 b). Mr. W. R. Dykes 4 has .pointed out that Iris spuria closely 
resembles /. Xiphium in floral characters. It is interesting to find that his 
idea of a possible relationship between these two species, and, through them, 
of a connexion between the Apogon and Xiphium Sections, is confirmed by 
the evidence of the leaf structure ; the limb of /. Xiphium (Fig. 32 b) is of 
a type which would be easily derivable from that of /. spuria (Fig. 8, p. 304). 
The facts of geographical distribution put no difficulty in the way of 
1 See Arber, A. (1918), Fig. 16 B, p. 483, for a more cylindrical leaf-type belonging to the same 
genus. 
2 Ross, H. (1892-3) 
3 Arber, A. (1920 1 ). 
4 Dykes, W. R. (1913). 
