332 Arber .— The Leaf Structure of the Iriciaceae , 
exclusively of leaf-base nature. In this Section the bristle-like inner 
perianth segments also suggest reduction-specialization. It should be 
recognized that the passage from the petiolar phyllode to the leaf-base leaf, 
though morphologically a reduction, may. from the standpoint of function , 
represent an advance. The leaf-bases of the Juno Irises, for instance, 
approach more nearly to true laminae, in structure and orientation, than 
do the petiolar phyllodes of the rest of the genus. The approximation to 
a normal Dicotyledonous blade, reached on this line of progression by the 
loss of the petiole and the exaggeration of the leaf-base, is arrived at on 
the second line of progression by a converse process — namely, elaboration 
of the petiole ; by increase of surface relatively to sectional area, the limb 
is modified into a pseudo-lamina, which is a more powerful instrument for 
performing the work of the leaf. This elaboration seems to depend 
essentially upon invagination or grooving, which may or may not be 
associated with the production of keels or wings. Grooving of the leaf- 
tissue between the bundles is highly characteristic of the family ; it occurs 
conspicuously in the leaf-base leaves of Crocus carpetanus (Fig. 59, p. 324) 
and Moraea bituminosa (Fig. 37, p. 31 5), as well as in various petiolar leaves. 
In the latter the invagination may be so slight as to .result merely in a series 
of ribs and channels, such as those seen in Iris xiphioides (Figs. 31 C and 
D, p. 313) and Romulea (Figs. 6 1-3, p. 324), or it may go much farther, and. 
as we have seen in the section on Foliated Leaves (pp. 320-3, and Figs. 50-3, 
p. 321), it may produce the curious pseudo-plicate winged leaves of Cypella , 
&c. Even the anomalous leaves of Crocus are susceptible, as I have shown 
on p. 325, of explanation on the same lines. The changes which take place 
in the petiolar phyllodes of the Irids, in their effort — if we may use the 
term — to become more effective assimilating organs, recall the peculiar 
stem developments which arise in the succulent Cactaceae and Euphorbias 
when the axis takes on the work normally accomplished by the leaf ; in 
both cases we find" corresponding morphological changes associated with 
the assumption, by a radial or approximately radial organ, of work which 
requires a large surface. The remarkable winged stem of Euphorbia 
grandicornis , which, as Goebel 1 points out, exposes a surface scarcely 
inferior to that of a leafy shoot, may be broadly compared with the foliated 
petiolar phyllode of Cypella or Herbertia , which forms an organ in which 
the ratio of surface to volume must be greater than in many normal 
laminae. 
The ingeniously diversified leaf-forms of the Iridaceae afford an 
illustration of the principle which I have termed the Law of Loss.^ On 
the phyllode theory, the leaf of the ancestral Irid, in common with other 
primaeval Monocotyledons, was of a type in which the lamina had been 
entirely eliminated, and, on the Law of Loss, this lamina having been once 
1 Goebel, K. (1889). 2 Arber, A. (1919 2 ). 
