350 Arber, — On the Nature of the ‘ Blade ’ in 
practically the whole petiole is involved ; e. g. various Iridaceae, such as 
Cypella - 1 
1 . From a dorsiventral petiole, or the distal region of a leaf- 
sheath : 
(a) Mainly by expansion, e. g. Rhipogonum (Fig. 1 6, p. 338). 
(b) By winging associated with expansion, e. g. Hydrocharis (Fig. 4, 
p. 331), Aponogeton (Fig. 3, p. 331). 
(c) By both dorsal and ventral invaginations associated with ex- 
pansion, e. g. Palms (Figs. 5 and 6, p. 334), Carludovica (Fig. 7, p. 334), 
Veratrum (Fig. 13, p. 338), Curculigo (Fig. 17, p. 341). 
(d) By numerous ventral invaginations, e. g. Psamma , and other 
Gramineae. 2 
(e) By a single ventral invagination associated with expansion and 
winging, e. g. Smilax (Figs. 14 and 15, p. 338). 
(f) By numerous dorsal invaginations, associated with expansion and 
winging, e. g. Dioscorea (Fig. 19, p. 341). 
(g) By two dorsal invaginations, associated with expansion and 
winging, e. g. Crocus . 3 
It should be noted that in the classification just outlined there is no 
hard-and-fast line between many of the subdivisions ; the value of such 
a grouping lies, rather, in the picture which it offers of the great range of 
final form produced from the distal region of the petiole (or sheath) 
by the interaction, in varying proportions, of a limited number of develop- 
mental factors. The most potent of these is invagination, and it is to the 
very various aspects under which invagination presents itself that the 
remarkable series of different forms assumed by the Monocotyledonous 
pseudo-lamina is mainly due. 
VI. Summary. 
As the result of a comparative study of blade development in the leaves 
of Monocotyledons (pp. 330-43 and Figs. 1-21) and Dicotyledons (pp. 344-6 
and Figs. 22-g) it is shown that while the ‘ blade ’ in some Monocotyledons 
follows a course of development indistinguishable from that of certain 
Dicotyledons, yet in other Monocotyledons a factor is involved which 
appears to play no part in the development of the leaves of Dicotyledons — 
namely, invagination of the tissues belonging to the petiole or sheath 
(pp. 347-9). A classification of the ‘laminae’ of Monocotyledons is suggested, 
based upon the factors concerned in their development. This classification 
brings out the fact that the great range of leaf form met with in these 
1 Arber, A. (1921 1 ), Fig. 50, p. 521. 
2 These cases will be described in a later paper on the Glumiflorae. 
3 Arber, A. (1921 1 ), Figs. 56 and 57, p. 324. 
