certain Monocotyledonous Leaves. 
345 
III. Observations on the Origin of the Lamina in the 
Leaf of certain Dicotyledons. 
Aristolochia. 
I have studied the structure of the young leaf of Aristolochia Clema - 
titis, L., because the cordate form of the lamina and the isolation of the 
bundles in the axis and petiole by broad medullary rays produce a certain 
similarity of type to some of the Monocotyledons we have been considering. 
The leaf at the point of detachment from the axis (ax., Fig. 22 A) has 
a sheathing base (s.) enclosing the axillary bud (b.). The three main leaf- 
bundles pursue an oblique course at this level, since they are close to their 
points of egress from the axis. The cordate lobes at the base of the lamina 
( c . and c'.) are, in the section figured, cut on either side of the leaf-base. 
A little higher (Fig. 22 b) the petiole (pet.) assumes its definitive form. 
Higher still (Fig. 22 C) we see the passage of the petiole into the lamina*, 
while Fig. 22 D shows the lamina itself. The ribbing of the underside recalls 
the slight invagination of the lower surface of the Tamus leaf (Fig. 18, p. 341). 
Polygonum. 
The leaf of Polygonum amphibium , L., to some extent resembles that 
of Potamogeton natans among the Monocotyledons, and for this reason 
I chose it for examination. Fig. 24 A shows a transverse section of an 
apical bud, passing through the sheathing leaf-base (f 1 s.) of the outer leaf 
{fi) and the base of the lamina (f 2 lam.)' and the ochrea (f 2 o.) of a second 
leaf, and also the lamina (f. z lam.) of a third leaf. Fig. 24 B shows the 
petiole (f.-ipet.) and ochrea (/. x 0.) into which the sheath of the first leaf 
seen in Fig. 24 A passes at a higher level. It will be noticed that there is 
a single series of bundles in the leaLsheath, while in the petiole and lamina 
there is also an inverted strand (i.b. 2 and i.b. z , Fig. 24 A, and i.b. v Fig. 24 B) 
opposite the median bundle. This inverted bundle arises by the fusion and 
subsequent branching of the lateral bundles of the sheath at a level a little 
below that at which the ochrea becomes free. Two of the laterals which 
take part in the- fusion are branches of the median bundle ( m.b . 1 ). On 
passing upwards into the limb, the bundles shown in the petiole of f x 
(Fig. 24 b) meet with the following fate : l. z fuses with /. 2 , and then 
f. x pet., petiole; m.b., median bundle; i.b., inverted bundle; / #1> /. 2 , /. 3 , l'. lf l'. 2 , t'. 3 , lateral 
bundles. Fig. 25, Polygonum cuspidatum , Steb. et Zucc., transverse section of apical bud (x 47), 
leaf sheaths and ochreas omitted, showing five leaves at successive levels and stages of development; 
f.-, petiole ; y !-/].!, development of coiled wings of lamina; no bundles differentiated except the 
median strand of f. x and f 2 . Figs. 26 A-F, Fagus sylvatica , L., series of transverse sections cut in 
autumn from leaves of next year’s bud, showing transition from petiole {pet., Fig. 26 a) to lamina 
(Fig. 26 f) ; not all from one leaf; vascular tissue shaded; xylem and phloem not distinguished 
Figs. 27 A-C, Syringa vulgaris, L., transverse sections showing transition from extreme base of petiole 
(Fig. 27 a) to lamina (Fig. 27 C) ( x 23). Figs. 28 a-f, Cercis Siliquastrum , L., series of transverse 
sections through one leaf (X23); Fig. 28 a, attachment of sheath, s., to axis, ax. ; Fig. 28 B, 
sheath, j. ; Fig. 28 c, detachment of stipules, st. and st'. ; Fig. 28 d, petiole, stipules omitted ; Figs. 
28 E and F, development of lamina, lam. Figs. 29 a jind B, Trifolium repens , L., sections through 
one leaf ( x 23) to show origin of laminae of leaflets {lam., Fig. 29 b) from petiolules {pet.. 
Fig.' 29 a). 
