L^x 
( 
43 
ill' cliloroph}^!^ l)ut very Itirge siihsidiary cells oeenin'ing half of the 
entire aixia. 
AsdmilatuKj lU.'iue. — The specialization of the epidermal cells of ferns 
is what their environment demands. In terrestrial species, with very few 
exceptions, they are not extremely differentiated from the parenchyma, 
but that this difference between the majority of ferns and the majority 
of spermaphytes is an adaptive, not a primitive character on the part 
of the fernSj is amply proved by the exceptions. Some species in every 
tribe of Polypodiaccm represented at San Ramon arc without chlorophyll 
in the epidermis. The spicular cells of the Yitiarlea' have already l)eeTi 
mentioned, and In four tribes, DaralUecr, Asplrniece, Pohjpodiece and 
Ackrostichea', are species which have carried protective specialization 
leeper than the epidermis, having a specialized hypodermis. In the 
majority of ferns it is more correct to describe the epidermis as specialized 
in other directions than for protection, than to call it nndiU'ercntiated. 
In very numerous ferns it is, indeed, a highly specialized photosynthetic 
tisstte which is not infrequently more specialized than any part of the 
parenchyma. 
Ferns with epidermal cells conspicuously deep and rich in chlorophyll 
are Cyclopcltis^ Nephrodium immersani, N. dive rail oh am, N. pteroides, 
Aspidium decurrens, A. angidatum, Stenosemia aurita, LepfocJiUus lati- 
f oil us, Diplazium pallidum, D. 1667, D. hnlhifernm, D. tenerum, T). 
Williamsi, Asplenium suhnormale^ A. resccium, A, scandms, ScliizosiegG 
calocarpa and Fteris quadriauriia. The inner ends of deep epidermal 
cells are out of contact Avith one another, leaving intercellular spaces, in 
Nephrodium. 1112, N. canescens, Meniscium, Microlepia pinnaia, Lind- 
saya liymenoplujlloides, L. ptdchella, Adiantum mindanaoenae. Poly po- 
dium ohliquatum and P. dolicliopterum. In several of these the 
chlorophyll is concentrated in the inner end. Instead of one, there are 
several inward projections, making the cells breeches-like in section in 
Nephrodium procurrem, N, Foxii, Stenosemia plnnata, Pohjpodium ra'fi- 
pitosum and P, cncullatum; the subepidermal layer has this character 
in Ilumata heterophylla. In Adiantum diaphanum, an especially large 
share in the i)hotoBynthesis falls to the epidermis, the upper and nether 
epidermis being in direct contact in a considerable part of the frond. 
It will be noticed that the ones which have been mentioned in tins con- 
nection are nearly all terrestrial species. But some very large terrestrial 
species are like many epiphytes in the more or less coinplete suppression 
of the chlorophyll in the epidermis, this being the case in Nephrodium 
ferox, N. ryafheoides, and the Imge variety of Aspidium leuzeanum. 
It is the mesophyll rather than the epidermis, which shows less special- 
ization in the ferns than in the seed-plants. In the parenclnma the 
differentiation is especially backward. A completely and typical de- 
veloped palisade parenchyma does not occur, but layers which are like it 
