certain Monocotyledonons Leaves . 337 
the development of the ‘ lamina’ (Figs. 11 C and d) take place in identically 
the same way. 
Pisiia. 
The leaf of Pistia Stratiotes , L., differs markedly from that of the three 
genera of Araceae already considered, both in its mature form and in its 
ontogeny. As I have described the leaf in a former paper, 1 and brought 
forward anatomical* evidence for regarding it as a petiolar phyllode, I will 
here confine myself to the question of the origin of the thin, lamina-like, 
distal region of the limb. The basal part of the limb is shown in Fig. 12 A ; 
it is solid and petiole-like. From Figs. 12 B and C it will be recognized 
that the development of the limb is due to a series of major dorsal 
invaginations and minor ventral invaginations, located between the main 
bundles, associated with lateral expansion of .the tissue connecting the 
bundles. 
Liliiflorae — L iliaceae. 
Veratrum . 
In a previous paper 2 I have studied the leaves of Veratrum album , L., 
in connexion with their prolonged cambial activity. The abbreviated axis 
bears annually a number of leaves, each with a cylindrical elongated leaf- 
sheath, succeeded by an ovate blade, which is plicated in its- young stages, 
and retains a channelled appearance throughout life. In order to follow 
the origin and development of the ‘ blade I cut serial transverse sections 
through the terminal underground leaf-bud of a plant cultivated under the 
name of Veratrum ‘ album nigrum \ This bud included a large number of 
leaves, but, for simplicity, I have only illustrated in Figs. 13 A-F the 
development of three of the younger leaves, which we may distinguish as 
f. 1, /. 2 , and f 3 . Fig. 1 3 A,, drawn from a section cut a little above the 
extreme base of the bud, showed the closed sheaths of the three leaves in 
question. We may choose the youngest leaf ,/l 3 , in which to follow the 
development of the ‘ lamina ’. In Fig. 13 A, although the limit of the closed 
sheath is not reached, and still more distinctly in Figs. 13 B and C, in which 
the passage to the blade is taking place, we observe that the leaf is losing 
its simplicity of form and is becoming deeply channelled by a series of 
invaginations (the more pronounced appearance of the invaginations on 
the right-hand side of the diagrams is merely the result of a slight 
obliquity in this series of sections). The positions of the grooves in the 
individual leaves is not haphazard, but follows a regular and definite plan. 
It will be recognized from Fig. 13 c that there is a dorsal invagination 
opposite each of the six main lateral vascular strands (l. v /. 2 , /. 3 , l'. v /'. 2 , /'. 3 ) 
and a ventral invagination between /. 3 and /. 2 , /. 2 and l. v /'. 3 and /'. 2 , l ' , 2 and 
l'. v and also one between /. x and l'. v i. e. opposite m.b. The result is that 
1 Arber, A. (1919 1 ). 
2 Arber, A. (1919 2 ). 
