as regards their
development.

=General morphology of the leaf and axis. Homology.= Since the time when
Goethe's generalisations were adopted by A. P. De Caudolle, special
attention has been given to the form and mode of development of the
leaf-organ; for as it was well said by Wolff, if once the course of
evolution and the structure of the leaf were known, those of the parts
of the flower would follow as a matter of course.

It is not necessary, in this place, to pursue the subject of the
development and construction of the leaf further than they are
illustrated by ordinary teratological phenomena.

From this point of view perhaps the most interesting circumstance is the
part that the sheath of the leaf plays.[553] In many cases of so-called
metamorphosis, it is the sheath of the leaf that is represented and not
the blade. In normal anatomy the sepals, petals, carpels, and even the
stamens, as a general rule, correspond to the sheath rather than to the
blade of the leaf, as may be seen by the arrangement of the veins. The
blade of the leaf seems to be set apart for special respiratory and
absorbent offices, while the sheath is in structure, if not in office,
more akin to the stem. It would not be easy apart from their position to
distinguish between a tubular sheathing leaf and a hollow stem. The
development of adventitious growths by chorisis or enation has been
frequently alluded to in the foregoing pages, and many illustrations
have been given of the power that leaves have of branching in more than
one plane, owing to the projection of secondary growing-points from the
primary organ. These new centres of development are closely connected
with the fibro-vascular system of the leaf, so that no sooner does a new
growing point originate, than vessels are formed to connect the new
growth with the general fibrous cord, see pp. 355, 445. This leads M.
Casimir De Candollo to consider the entire leaf as a composite
structure. The morphological unit, says he, is the cellular protrusion
or growing point (_saillie_) and its corresponding fibro-vascular
bundle.[554]

The identity, in a morphological point of view, of the leaves and the
lateral parts of the flower is so thoroughly recognised that little
need be said on that score, save to repeat that the homology of the
floral organs is usually not so much with the entire leaf as with its
sheath.

The most singular instances of morphological identity are those relating
to