38 
gaimardiana, IT. parvula (fig. 3), Davallia brevipes (fig. 4), D. solida, 
Asplenium vulcanicum, A. macro phy Hum, A. cuneatum, A. Belangeri, 
Pteris ensiformis, Monogramma, A/ntrophyum latifolium, Polypodium 
cucullatum and P. Pliymatodes. In Hurnata gaimardiana and Davallia 
brevipes these pores reach a depth of three-fourths to four-fifths of the 
thickness of the wall. In leaving these lobes of thick walls thin, the plant 
makes the most economical mechanical use of its plastic material, for the 
lateral walls effectively reinforce the outer ones at the edges. Straight, 
lateral walls act less effectively in the same way and a few ferns have the 
outer Avails thinner around the even margin, as is the case in Nephrodium 
1712, or they are pitted around the margin, as in Asplenium squamulatum 
(fig. 5). On the other hand, Scolopendrium pinnatum, the epidermal 
cells of which are rather large in proportion to the thickness of the outer , 
and lateral walls, has the outer wall reinforced eentripetally for a short 
distance from each of the obscure entrant lobes (fig. 6). The same is 
true of some of the entrant angles in A thyrium silvaticum, whereas Asple- 
nium scandens has the outer wall thinned in the excurrent lobes and 
reinforced from the entrant curves. 
The “spicular” idioblasts of the Vittariece (figs. 7, 8) have long been 
familiar objects. They are wanting in Antropliyum latifolium and 
Vittaria minor, the former growing in sheltered places and being too 
broad to need any longitudinal reinforcement, and the latter being very 
small and stout. 
It has just been pointed out that wavy, lateral walls are a reinforce- 
ment of the outer wall, contributing greatly to the rigidity and strength 
of the epidermal framework as a whole. On other than mechanical 
grounds, most terrestrial ferns have the outer walls much thicker above 
than below. In evident correlation with this fact, these ferns almost 
invariably have wavy walls for the nether epidermis alone, or have the 
latter decidedly more wavy than those above. Diplazium furnishes many 
good illustrations. Wavy, lateral walls, by reinforcing the outer walls 
or in extreme cases, dividing the cell into comparatively small lobes or 
parts, make larger epidermal cells possible than would' otherwise be 
tolerated from mechanical reasons. The relation of size of cell and 
waviness of wall to the stomatal movement will be discussed presently. 
The relation of the thickness of outer wall to size of cell needs no 
argument; it is well illustrated within one plant by Odontosoria retusa, 
which, though a terrestrial fern along creeks, has the outer wall 1.5 p 
thick above, and 3 p to 4 p thick below, the cells above being remarkably 
narrow. 
Stomata. — Stomata occur on the upper surfaces of the fertile fronds of 
Achrosticlium and Clieiropleuria and on all four faces of those of Mono- 
gramma ; otherwise they are entirely confined to the nether surfaces of all 
