Si ok 
DEVELOPMENT OF LEAVES. 811 
the leaf separates,’ as Inman says, ‘the tree does not suffer 
from the effect of an open wound.’ Gray then, quoting Inman, 
adds :—‘ The provision for the separation being once complete, 
it requires little to effect it ; a desiccation of one side of the 
leaf-stalk, by causing an effort of torsion, will readily break 
through the small remains of the fibro-vascular bundles ; or the 
increased size of the coming leaf-bud will snap them ; or, if 
these causes are not in operation, a gust of wind, a heavy 
shower, or even the simple weight of the lamina, will be 
enough to disrupt the small connexions and send the suicidal 
member to the grave. Such is the history of the fall of the 
leaf.’ 
(8) Development of Leaves.—Leaves and all their homologous 
forms, such as the parts of flowers, &c. are developed late- 
rally just below the apex of the stem by cell-division either of 
a group of cells as in the Phanerogams, or of a single cell as 
in the Vascular Cryptogams. A conical papilla, or (in sheath- 
ing leaves) an annular collar, is then the result of a deflection 
to one side of a group of these divided cells. Leaves are 
formed acropetally or indefini'ely, the youngest always being 
the highest, according to the laws of Phyllotaxy. ‘The papille 
from which the leaves originate are at first wholly cellular, con- 
sisting of periblem or proto-meristem, covered by a layer of 
dermatogen cells ; after a time elongated cells are formed in the 
centre ; and these are followed by spiral vessels, formed in a 
direction from the base upwards.’ The first formed part of the 
leaf generally corresponds with its apex or with the summit of 
the common petiole—i.e. the apex of a leaf is generally its oldest 
instead of its youngest part as is the case with the stems where 
the apex is the growing point. (See page 795.) In leaves the 
apical growth soon ceases, though interstitial growth continues. 
The following is an abstract of Trécul’s conclusions :— 
‘All leaves originate in a primary cellular mamilla, with or 
without a basal swelling, according as they are to have sheaths 
or not; they are developed after four principal types : (1) the 
centrifugal formation, from below upwards ; (2) the centripetal 
formation, from above downwards ; (3) the mixed formation ; and 
(4) the parallel formation. The centrifugal or basifugal develop- 
ment may be illustrated by the leaf of the Lime-tree, which 
begins as a simple tumvur at the apex of the stem. This tumour 
lengthens and enlarges, leaving at its base a contraction which 
represents the petiole. The blade, at first eutire, is soon divided 
from side toside byasinus. The lower lobe is the tirst secondary 
vein. The upper lobe is divided in the same manner five or 
six times, forming as many secondary veins. Sinuosities then 
appear in the lower lobe, indicating the ramifications of the 
lower vein ; and, finally, fresh toothings appear corresponding 
with more minute ramifications. Thus the various veins in the 
leaf of the Lime-tree are developed like the shoots of the tree 
