344 Bower . — The comparative examination of the 
menophyllaceae, the reniform lamina being composed of four to 
six layers of cells, though it preserves the filmy character in as 
much as stomata and intercellular spaces are absent. Prantl 
conjectures shrewdly enough 1 that the origin of these many 
layers is by segmentation similar to that described by him for 
the origin of the nerves, but he had not the necessary material 
for proving this : the following statement will show that his 
conjecture was sound. It has been shown how the leaf of 
Trichomanes reniforme is provided with a marginal series of 
cells (Fig. 33) ; these give rise to the wings which run down 
the phyllopodium to its base : upwards the leaf widens out into 
the broadly reniform lamina. On cutting transverse sections 
of the lower part of the young leaf, the wings are seen as two 
round projections, in which the marginal cells do not undergo 
transverse segmentation, but the segments are cut off ob- 
liquely and alternately from opposite sides (Fig. 50) : the 
number of segments thus produced is however small, and the 
wings of this part of the leaf are never largely developed. 
Now this mode of segmentation of the marginal cells is 
characteristic of the Polypodiaceae and other leptosporangiate 
Ferns, and is, to say the least, extremely rare in the Hy meno- 
phyllaceae: even here it is only seen near the base of the leaf, 
where, as in other Ferns, the structure of the wing is habitually 
more robust than in the upper parts 2 . 
Transverse sections of the reniform lamina of the mature 
frond of Trichomanes reniforme show that in the parts be- 
tween the nerves it consists of four to six layers of cells, of 
which the central layers are larger and more watery, while the 
superficial cells have more protoplasm and chlorophyll : no 
intercellular spaces nor stomata are present. If transverse 
sections be cut from the young leaf a marginal cell of large 
size is seen in each, having a form similar to those found 
in the thin frond of Trichomanes radicans , etc. (Fig. 51) 3 : 
1 l. c. p. 23. 
2 See below, p. 350. Compare Aspidium Filix-mas, Osnmnda, Todea, and 
Marattiaceae. 
3 Compare Fig. 47 of Trichomanes radicans , which is drawn to the same scale 
as Fig. 51. 
