333 
considered in Relation to the Phyllode Theory . 
discarded, could never be regained. A review of the Irid leaves leads 
us to the conclusion that there has been, as it were, an effort in this 
family to find some substitute for the lost lamina which would be more 
efficient than the typical petiolar phyllode as an organ of assimilation. 
The members of the family have all had the same problem to solve, and 
the same material — -the petiolar phyllode- — to work upon ; they have 
apparently discovered no methods by which to deal with the situation, 
except invagination and the development of keels or wings. But we have 
only to compare the mature leaves of such divergent types as Gladiolus tristis 
(Fig. 48 J, p. 318), Crocus (Fig. 56 1 , p. 324), and Cypella (Fig. 50 N, p. 321), 
to see what strikingly different results have been attained by the various 
modes in which these monotonous methods have been applied. 
In surveying the whole range of Irid leaf forms, one can scarcely fail 
to be struck by the more or less parallel developments from the ensiform 
type which have recurred within different tribes and genera. In the 
Iridoideae we meet, in the genus Iris, with ensiform and ‘ radial ’ phyllodes, 
and also with leaf-base leaves ; with both ensiform and cylindrical phyllodes 
in each of the genera Sisyrinchium , Trimezia, and Bobartia ; with foliated 
leaves in Cipura , Cypella , and Tigridia. In the Ixioideae there are many 
ensiform leaves, such as those of Schizostylis ; ensiform leaves and also 
approximately radial phyllodes in Gladiolus ; foliated leaves in Babiana . 
A corresponding series of ‘ Parallelbildungen ’ has been recognized in the 
different genera of Cactaceae ; Ganong 1 enumerates a number of characters 
which recur repeatedly in different lines within the family. One is reminded 
also of the £ phyletic drift’ recognized by Bower 2 in the case of the 
ferns. 
io. Summary. 
In the present paper a general examination of the leaf structure of the 
Iridaceae is undertaken, in order to see how far the view can be sub- 
stantiated that the leaf in this family is a phyllode , possessing no lamina 
but consisting of petiole and leaf -base, or leaf -base alone . It is found that 
a study of the transition region between sheath and limb often throws light 
on the morphology of the leaf. 
The ensiform (equitant isqbilateral) leaf is first discussed, and the 
conclusion is reached that the evidence — especially the comparison with 
modified leaves in the genus Acacia — negatives the congenital concrescence 
theory, and points to the ensiform leaf being a petiolar phyllode (pp. 303-6). 
The semi-equitant leaves of Phormium and Dianella (Liliaceae) are held 
not to be comparable with the ensiform leaves of the Iridaceae, but to have 
attained their curious form as a secondary modification (pp. 306-7). It is 
shown that not only are ensiform leaves in many respects similar to Acacia 
1 Ganong W. F (1894) 2 Bower, F. O. (1918). 
Z 
