76 Campbell.— On the Prothallium and Embryo of 
The cotyledon does not break through the calyptra until a 
late stage, and in this respect, as well as others, shows its 
primitive character. After the cotyledon has broken through, 
it lengthens rapidly, but the increase in length, as well as the 
subsequent increase in thickness, is due largely to the simple 
enlargement of the cells. The petiole grows out toward the 
edge of the prothallium until it reaches it, and then grows 
upward, and then the lamina, which has been sharply bent 
backward against it, with the inner (upper) surfaces of the two 
lobes in contact, gradually flattens out. It, as well as the 
petiole, is almost perfectly smooth, the glandular hairs seen in 
the young stages having pretty much disappeared. The lamina 
is obscurely two-lobed. The single fibro-vascular bundle of 
the petiole forks at the base of the lamina, and each of the 
two branches divides again, and sometimes these branches 
divide once more (Fig. 98). Corresponding to the division, 
the margin of the primary lobes is indistinctly lobed. 
A cross-section of the petiole of the full-grown cotyledon 
(Fig. 1 01) is strongly convex upon the outer side, and nearly 
flat upon the inner surface. The epidermis is composed of 
strongly convex cells, not noticeably different from those of 
the ground-tissue below it. The latter are somewhat irregular 
in outline (in the figure this is somewhat exaggerated through 
slight shrinkage of the inner ground-tissue in the process 
of imbedding), and become smaller as they approach the 
single fibro-vascular bundle in the centre. This is nearly 
circular in outline, and lies nearly in the centre, but slightly 
nearer the inner side. It is pretty clearly defined, but a true 
bundle-sheath is hardly to be distinguished. It is composed 
of nearly uniform thin-walled cells, some of which are probably 
to be regarded as sieve-tubes, but not clearly recognizable as 
such. Toward the inner side, and separated from it only by 
one or two rows of cells, is a group of small tracheides (ay), 
which, like all of the earliest ones, are marked with close 
annular or reticulate thickenings. The bundle, while perhaps 
to be regarded as concentric, approaches closely, in the 
arrangement of its elements, the collateral bundle of Ophio- 
