MORPHOLOGY. 477 
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 sub- 
ject of the development and construction of the leaf - 
further than they are illustrated by ordinary terato- 
logical phenomena. 
From this point of view perhaps the most interesting 
circumstance is the part that the sheath of the leaf 
plays.’ 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, corre- 
spond 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 con- 
nected 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 Candolle to consider the 
entire leaf as a composite structure. The morpho- 
logical unit, says he, is the cellular protrusion or 
erowing point (saillie) and its corresponding fibro- 
vascular bundle.* 
The identity, in a morphological point of view, of 
the leaves and the lateral parts of the flower is so 
1 See Clos., ‘Bull. Soc. Bot. Fr.,’ 1856, vol. iii, p. 679. 
> «Théorie de la Feuille,’ p. 26. 
