348 Arber . — On the Nature of the ‘ Blade ’ m 
But the uniformity of development met with in the Dicotyledons which 
we have described, is not paralleled among the Monocotyledons. Besides 
those cases to which we have just alluded, which approximate to the 
Dicotyledonous type, we have a number of others in which invagination of 
the petiolar tissues plays a part of greater or less importance in the formation 
of the blade. Such cases have a, remarkably wide distribution through the 
Class. I have described examples in the present paper from the Palmae, 
Cyclanthaceae, Araceae, Liliaceae, Amaryllidaceae, Dioscoreaceae, and 
Iridaceae, 1 and I have illustrated the origin of the * lamina ’ by invagination 
in the case of Areca (Fig. 5, p. 334), Oreodoxa (Fig. 6, p. 334), Carludovica 
(Fig. 7, p. 334), Pistia (Fig. 12, p. 336), Veratrum (Fig. 13, p. 338), Smilax 
(Figs. 14 and 1 5, p. 338), Curculigo (Fig. 17, p. 341 ),Dioscorea (Fig. 19, p. 341), 
and Tigridia (Fig. 21, p. 341). The process is perhaps most strikingly 
displayed in the Palms and Irids ; in many cases it produces a ‘plicate’ 
form of blade. How fundamentally this ‘ plication ’ differs from the genuine 
folding met with in some Dicotyledons, will be realized on comparing 
Figs. 5 A-E, p. 334 (illustrating the formation of the plicate limb of the Palm, 
Areca sapida , by alternating invaginations which penetrate between the 
vascular strands of the petiole), with Figs. 26 A— F, p. 344 (illustrating the 
development of the Beech-leaf, Fagus sylvatica , in which the two halves 
of the lamina fold up in a delicately fan-like fashion as they increase in 
width). As another pair of contrasting cases we may take Crocus and 
Polygonum ; in the Crocus- leaf 2 a couple of dorsal invaginations finally 
produce a revolute vernation closely similar to the arrangement of the coiled 
lateral wings of the leaf of Polygonum (Fig. 24, p. 344). 
Though I have not observed invagination as a main factor in lamina 
development either in the Helobieae or in the Araceae (except Pistia ), yet 
in the cases which I have examined from these groups in which the leaves 
are definitely auricled at the base ( Sagittaria of the Alismaceae, Calla , 
Arum , and Epipremnum of the Araceae) I have found that the auricles are 
detached by the penetration of a deep groove, which should perhaps be 
classed with the invaginations met with elsewhere among Monocotyledonous 
leaves. These grooves in Sagittaria start from the dorsal surface (see arrows 
in Figs. 2 D and H. p. 331), whereas in Calla, Arum , and Epipremnum they 
penetrate into the leaf from the ventral side (Figs. 8 E, 9 D, and 11 B, p. 336). 
From the comparison which we have instituted between the develop- 
ment of Monocotyledonous and Dicotyledonous laminae, I think we may 
conclude that, although in a number of Monocotyledonous leaves ‘ blade * 
development proceeds on lines closely similar to those followed in the case 
of the true laminae of Dicotyledons, yet in some cases whole families 
(e. g. Palmae) and in other cases individual genera within a family 
1 The Gramineae, which will be described in a later paper, may be added to this list. 
2 Arber, A. (1921 1 ), Figs. 56 and 57, p. 324. 
