358 
DUNCAN S. JOHNSON 
making up the wall of the ovary, only the innermost and outermost are 
appreciably specialized in structure. The only exception to this is the 
single delicate strand of vascular tissue running down the abaxial side 
(figs. ICQ, loi, io8). This strand is well-developed by the time the 
embryo sac is mature (fig. 78). It consists from this time on of but 
2 or 3 ringed ducts, 3 or 4 in diameter, and of a few small, elongated 
cells nearby, that may play the part of phloem (figs. 100, loi, 108.) 
This simple vascular bundle connects with the lattice-like tracheids of 
the style above, and with the vascular bundle of the stalk of the fruit 
below. Aside from these vascular tissues, the 2 to 5 cell-layers of the 
carpel between its inner and outer layers, consist of but slightly elon- 
gated, thin-walled parenchymatous cells (figs. loi, 108). 
Up to a late stage in the development of the fruit, the cells forming 
the innermost layer of the carpel remain thin-walled, and each may 
contain two or three dozen chloroplasts. In the mature fruit, however, 
the inner walls of these cells and the inner portions of all four radial 
walls become slightly thickened, and the whole cell contents turn brown 
and shrink against the inner walls of the cell (fig. 108). The chloro- 
plasts of these cells, though shrunken and brown, do not lose their 
identity, as they do in, the dried contents of these same cells in Feper- 
omia pellucida (Johnson, 1900a, fig. 15). Neither are the walls of 
these cells thickened to the same degree, nor in the complex lattice-like 
pattern of those of P. pellucida. 
The outer epidermal layer of the carpel is far more highly special- 
ized than any other. It consists, when mature, of four distinct types 
of cells, the oil-cells, the hydathodes, the bristle-bearing cells and the 
ordinary less modified epidermal ones. The latter differ in size and 
shape (figs. 107, 108). Their convex outer walls are slightly thickened 
with a corrugated cuticle, like that of the epidermis of the stem, and 
leaf (cf. fig. 13). The oil-cells are structurally the least modified of the 
other surface cells of the fruit. They are nearly isodimensional, and 
are commonly shaped like truncated pyramids with their bases inward 
(figs. 78, 100, 107, io8a). Up to the time of the maturing of the em- 
bryo sac in an ovule, the protoplast in each of the oil-cells of its carpel 
is thick and the nucleus and vacuole about half the diameter of the 
cell. In the mature fruit the protoplast is usually somewhat shrunken 
and, in unstained material, only a few small yellowish granules are 
visible. Imbedded in the part of the protoplast lying against the 
outer wall, however, is a disk of a substance that stains a dense black 
with iron haematoxylin (figs. 100, 108, 1 08a). 
