1922] ARBER—LEAVES OF FARINOSAE gI 
leaf of Philydrum (fig. 22) may be compared with the “radial”’ 
leaf of the related genus Pritzelia (fig. 21). 
The type of leaf consisting of a sheathing leaf base terminating 
in a more or less cylindrical apex, which I interpret as a reduced 
petiole, occurs both in the Farinosae and Liliiflorae. For instance, 
the leaf of Elegia deusta Kth. (Restionaceae) is closely similar to 
that of Distichia clandestina Buch. of the Juncaceae (10). The apical 
tendril of Flagellaria (Flagellariaceae). also recalls that of Gloriosa 
and other Liliaceae. I hope to discuss the morphology: of these 
leaf tendrils in a later paper. 
To complete the parallel between the leaves of the Farinosae 
and those of the Liliiflorae, it may be noted that the limbs of the 
Pontederiaceae and of Helmholtzia, with their inverted bundles, to 
some extent approach those of certain species of Allium (5), and 
of such Amaryllids as Zephyranthes (8); while the similarity in 
shape of the cordate leaf limb of the Commelinaceous climber, 
Streptolirion volubile (fig. 9), and that of various Dioscoreaceae and 
Liliaceae is of wider interest, since it is an example of the recur- 
rence of a form which appears again and again among monocotyle- 
dons. In former papers (1, 4) I have brought together a number 
of instances, from this class, of leaves with a cordate base; the 
list of families in which leaves of this type are found may now be 
increased to ten by the addition of the Stemonaceae, Amaryllidaceae, 
and Hydrocharitaceae. This comparison between the leaves of the 
Farinosae and those of the Helobieae and Liliiflorae emphasizes 
again the important part which parallelism of development has 
played in the evolution of the monocotyledonous leaf. The 
tendency for related stocks (even those whose affinity is far from 
close) to progress along corresponding lines is no doubt a very 
general feature of evolutionary history, although its prevalence is 
only gradually receiving full recognition. The fact that, in the 
monocotyledonous leaf, such parallelisms are displayed to an 
almost exaggerated degree, becomes to some extent explicable, if 
this organ is regarded as a phyllode consisting of leaf base and 
petiole alone. The loss of the lamina would inevitably impose 
restrictions upon the further evolution of the leaf, by confining 
its potentialities within a narrowed boundary. It might thus 
