328 Bower . — The comparative examination of the 
have laid the foundation of a knowledge of the mode of 
development of the fronds of Ferns : also in the Hydropterideae 
the development of the leaf has been carefully traced in 
Marsilia , Salvinia , and Azolta : I have also shown that the 
leaf of Pilularia originates in a manner similar to that of typical 
Ferns 1 . Notwithstanding all this there is still an open field 
for comparative observation on the mode of development of the 
leaf in different members of the Filicineous series : when it is 
remembered that the general character of the Fern plant is 
more closely dependent upon that of its leaves than on any other 
part, the omission to subject it to a more exact developmental 
study appears remarkable. The same mode of treatment 
will be applied here as in the root and stem : the leaf of the 
Hymenophyllaceae will be first considered, then successively 
those of other leptosporangiate Ferns, of Osmundaceae, and 
Marattiaceae. It will be assumed to be unnecessary to re- 
capitulate the general results of other observers, and a 
reference to the writings above cited, and to the precis of 
the subject given by Sadebeck 2 will be sufficient. 
Only very scanty facts are at present known as regards the 
origin of the frond of the Hymenophyllaceae, and for these 
we are chiefly indebted to Prantl 3 , who has pointed out that 
in Trichomanes speciosum the leaf has a two-sided apical cell, 
and that from the segments a marginal series of cells is derived 
on either edge of the leaf. These points I have found to be 
correct for other species. I have observed early stages of 
development of the leaves in Trichomanes radicans , and find 
that they arise laterally upon and alternately on opposite 
sides of the axis : no exact numerical relation has been 
recognised between the leaves and the segments cut off from 
the apical cell of the stem, and it would appear that many 
segments pass without taking part in the formation of leaves. 
These arise from the lower edges of the obliquely lateral 
segments (Fig. 19 /), and appear as convex projections in 
1 Phil. Trans. Part II, 1884, p. 574. 2 Schenk’s Handbuch, I, p. 270. 
3 Hymenophyllaceae, pp. 4-7. 
