346 Arber . — On the Nature of the ‘ Btade ’ zVz 
(/. 3 + /. 2 ) fuses with /. r A comparison of the petiole (f^pet.) in Fig. 24 B 
with the two laminae cut at different levels in Fig. 24 A (f. 2 lam. and f . 3 lam.) 
shows that the lamina arises as lateral marginal outgrowths from the petiole, 
which become coiled as they develop. 
In Polygomim cuspidatum , Sieb. et Zucc., the development of the 
lamina is essentially similar. Fig. 25 shows part of a transverse section 
of a very young leaf-bud of this species, in which, for simplicity, the leaf- 
sheaths and ochreas are omitted. In the leaf f . 5 the petiole is cut across ; 
in the leaves /. 4 and f ‘. 3 the lamina is seen arising as a pair of marginal 
wings ; in the leaves f . 2 and f. v which are each old enough to show 
a lignified median strand, the wings of the lamina are elongating and 
beginning to coil. 
Populus . 
Serial sections of the bud of a cultivated species of Poplar (probably 
Popidus nigra , L., or a related form) show, at the base of the leaf, the 
stipules and petiole united into a sheath (Fig. 23 A). Higher up the 
stipules (s. and s'.) and petiole (pel.) become free (Fig. 23 B). The two 
halves of the lamina develop — as in P olygonum — as outgrowths from the 
margins of the petiole (Fig. 23 C), which become spirally coiled (Figs, 23 D 
and E). But whereas the rolling is revolute in Polygonum it is involute in 
Populus. 
Fagus. 
Sections of young leaves, dissected in October from a next year’s bud 
of the Beech, Fagus sylvatica , L., show that the lamina develops as a pair 
of flat wings, which fold in a fan-like manner as they increase in area 
(Figs. 26 A-F). 
Cercis. 
The simple, almost cordate, blade of the Judas-tree, Cercis Siliquastrum , 
L., has a sheathing base (.?., Figs. 28 A and B) from which stipules (st. and 
st'.) detach themselves (Fig. 28 c). The petiole shows its characteristic 
form in Fig. 28 D, while in Figs. 28 E and F the halves of the lamina are 
seen arising from the petiole as upwardly directed outgrowths. 
Trifolium . 
The laminae of the leaflets of the compound leaf of Trifolium repens , L. 
(Fig. 29 B), develop as outgrowths of the partial petioles (Fig. 29 a), very 
much as the blades of the simple leaves of Cercis arise from their leaf- 
stalks. 
Syringa. 
The Lilac, Syringa vidgaris , L., is an example of a leaf in which the 
petiole (Fig. 27 A) passes almost insensibly into the lamina (Fig. 27 c). 
