320 Arber . — The Leaf Structure of the Iridaceae , 
• 
but representing merely the dorsal or ventral crest of a flattened petiole — 
naturally tends more to massiveness. 
Not only marginal but also apical fibrousness is a common character 
of Irids and Acacias; the indurated apices of certain ‘radial’ Acacia 
phyllodes may be compared with the horny leaf-tips of Iris reticulata . 
It is scarcely possible to cut many sections of the strongly fibrous 
leaves of Acacias and Irids without being impressed by the apparently 
excessive character of the fibrosis — excessive, that is to say. from the point 
of view of any purpose which it may conceivably serve in the plant’s 
economy. The most reasonable view appears to be that the fibrosis is 
a process outside the plant’s control, which may almost be treated as 
a pathological result of unsuitable conditions. Dr. A. H. Church 1 has 
recently suggested that the xerophyte is, as it were, embarrassed by excess 
of waste polysaccharides, and deposits them on its cell-walls simply to get 
rid of them. The conditions which bring about this excess are, on his 
view, the disturbance of the balance between photosynthesis and proteid 
synthesis, due to the inadequacy of the transpiration stream in relation to 
the available sunlight. The common association of fibres and phloem, and 
the fact that fibrosis tends to increase towards the leaf apex (e. g. Crocus 
vermis, Fig. 57 B and C., p. 324, and Hermodactylus tuberosus , Fig. 29 E, 
p. 311), may perhaps be regarded as affording some indirect support to 
Church’s view. If his theory be accepted, any utility which may be 
attributed to the fibres must be regarded as an entirely secondary 
matter. 
6. Foliated Leaves in the Iridoideae and Ixioideae. 
Some of the most aberrant leaf-types met with in the Iridaceae are 
those for which the names ‘folia tabulata 5 and ‘ mehrflachige Blatter’ 
have been suggested, and which I propose here to term ‘foliated leaves’. 
As Lindman 2 rightly points out, these leaves do not differ fundamentally 
from the ensiform type, though they have been in many cases modified 
almost out of recognition. The least modified form is perhaps that met 
with in Babiana (Ixioideae). Fig. 50 A shows a typical plant belonging 
to this genus. The earlier leaves (rd. v rd . 2 , r./. 3 ) consist chiefly of sheath, 
with a minute limb in the case of r.l. 2 and r./. s . In the typical foliage 
leaves l. v /. 2 , and /. 3 the limb becomes more and more conspicuous, while 
the sheath at first sight suggests a petiole. It is by no means rare in 
Monocotyledons to find that the ancestral loss of the leaf-blade, and the 
compensatory development of the petiole as ( a ‘pseudo-lamina’, 3 is 
associated with the assumption by the sheath of petiole-like functions. 
Fig. 50 G shows the truncated form of the limb in another species, 
1 Church, A. H. (1919). 2 Lindman, C. A. M. (1899). 3 Arber, A. (1918). 
