certain Monocotyledonous Leaves. 
349 
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(e. g. Veratrum and Smilax in the Liliaceae) owe the production of their 
‘blades’ to invagination of the distal region of the petiole or sheath, 
a process which does not — so far as my observations go — occur as a prime 
factor in the leaf development of Dicotyledons. These cases of invagination 
are so numerous and so widely scattered through the Class as to suggest 
that they have a definite phylogenH:ic significance, and that they represent 
a morphological tendency inherent in the group as a whole. I think we 
are justified in regarding the part played by invagination in the development 
of the blades of Monocotyledons as offering some confirmation of the view 
that these organs are pseudo-laminae. 
V. Classification of the ‘ Blades ’ of Monocotyledonous 
Leaves. 
I propose, in the present section of this paper, to assume the truth 
of the phyllode theory, and to attempt a classification of the pseudo-laminae 
of Monocotyledonous leaves, based upon the factors to which they owe 
their development from the distal region of the petiole (or sheath). These 
factors are : 
(i) flattening ; 
(ii) expansion, associated with separation of bundles ; 
(iii) formation of wings or keels, which may be marginal only, or which 
may involve other regions of the leaf ; 
(iv) invaginations, which may be single or numerous, and which may 
penetrate into the leaf from either the dorsal or ventral surface, or both. 
The ultimate form of the ‘ blade ’ depends, in the first instance, upon 
the form of the petiole from which it is derived, and, secondly, upon the 
developmental factor (or combination of factors) responsible for blade 
formation. On this basis the pseudo-laminae of Monocotyledons may be 
grouped as follows, according to their mode of development: — 
i. From a petiole of more or less radial structure : 
(a) By flattening and expansion alone, so that radial anatomy is 
retained. 
(i) distal region of petiole only involved, e. g. Pontederiaceae. 1 
(ii) whole petiole involved, e. g. Allium victoriaUs? 
(b) By flattening and expansion, so that radial anatomy is retained, 
associated with dorsal and ventral invaginations, e. g. Pistia z (Fig. 12 , 
P- 33 6 )- 
(c) By invaginations, associated with expansion and winging ; the 
invaginations, though morphologically dorsal, penetrate the petiole laterally ; 
1 Arber, A. ( 1918 ), pp. 489-91 and Fig. 23-30. 2 Arber, A. ( 1920 3 ), Fig. 25, p. 457. 
3 Arber, A. ( 1919 *). 
C c 
