352 Davis. — On the Development of the Frond of 
probably of a plant in about this same stage of development. 
The sections h , c , and d of this figure show on the inside cross- 
views of the hyphae. 
Perhaps the most instructive figure is Fig. 18. The plant 
from which this was taken was -28 mm. in length. In this the 
initial cells are lettered x and x *, and the segments derived 
from each initial cell are numbered to correspond. The 
typical arrangement of the segments and of the cells which are 
cut off from them to form the hyphae is, I think, apparent. 
Segment i 1 has not cut off its hyphal cell ; all the rest have. 
In the older segments (4, 5, 3 1 and 4 1 ) the cortical portion of 
the segments has again divided, and this illustrates very beau- 
tifully the manner in which the cortex grows. 
In this specimen, Fig. 18, there is one well-developed 
diaphragm, lettered d 1 , and also the beginnings of the second 
diaphram, lettered d 2 . At this last point some of the hyphal 
cells have met and become fastened together. By the farther 
division and growth of these hyphal cells, keeping pace with 
the widening of that portion of the frond, the second diaphragm 
would have been developed. One bulb-cell, lettered b> is shown 
in this section, attached to a hypha (h). The development of 
the bulb-cells has not been seen in these young plants, but 
there is no doubt, from the investigations of Debray and 
Bigelow, that they arise from hyphae. 
I wish again to call attention to the lateness of the develop- 
ment of the hyphae. The cap-cells of the segmented spore 
often divide several times before the hyphae appear. That 
this is so is proven by the fact that the initial cells of a young 
plant are much smaller than the cap-cells of segmented spores. 
Compare Figs. 14 and 15, both drawn under the same magni- 
fication ; the segmented spore (Fig. 14) is *08 mm. long, the 
young plant (Fig. 15) is -20 mm. long. The hyphae, when 
they first appear, are very small (see Fig. 16) and insignificant. 
If the hyphae are to be considered as structures which are 
directly derived from the initial cells and which give rise to 
the cortex of the adult frond, one would hardly expect to find 
them to be the last structures formed in the development of 
