43 
in chlorophyll, but very large subsidiary cells occupying half of the 
entire area. 
Assimilating tissxie. — 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 ferns, is amply proved by the exceptions. Some species in every 
tribe of Polypodia real represented at San Ramon are without chlorophyll 
in the epidermis. The spicular cells of the Vittariece have already been 
mentioned, . and in four tribes, Davalliece , Aspleniece, Polypodiece and. 
Achrostichea , are species which have carried protective specialization 
deeper 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 undifferentiated. 
In very numerous ferns it is, indeed, a highly specialized photosynthetic 
tissue which is not infrequently more specialized than any part of the 
parenchyma. 
Ferns with epidermal cells conspicuously deep and rich in chlorophyll 
are Cyclopeltis, N ephrodium immersum, N. diversilobum, N. pteroides, 
Aspi'dium decurrens, A. angulatum, Stenosemia aurita , Leptochilus lati- 
folius, Diplazium pallidum ■, J). 1667 , D. bulbifermn, D. tenerum, D. 
Williamsi, Asplenium subnormale, A. resectum, A. scandms, Schizostege 
calocarpa and Pteris quadriaurita. The inner ends of deep epidermal * 
cells are out of contact with one another, leaving intercellular spaces, in 
N ephrodium 1712, N. canescens, Meniscium, Microlepia pinnata , Lind- 
saya hymenophylloides, L. pulchella, Adiantum mindanaoense, Polypo- 
dium obliquatum and P. dolichopterum, 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 
N ephrodium procurrens, N. Foxii, Stenosemia pinnata, Poly podium cces- 
pitosum and P. cucullatum; the subepidermal layer has this character 
in TLumata heterophylla, In Adiantum diaphanum, an especially large 
share in the photosynthesis 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 this con- 
nection are nearly all terrestrial species. But some very large terrestrial 
species are like many epiphytes in the more or less complete suppression 
of the chlorophyll in the epidermis, this being the case in N ephrodium 
ferox , N. cyatlieoides , and the huge 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 parenchyma the 
differentiation is especially backward. A completely and typical de- 
veloped palisade parenchyma does not occur, but layers which are like it 
