435 
From the species I have examined it appears, first, that in any 
one genus the number of cells in thickness of the mesophyl is 
constant; Adiantum, 2; Dryopteris, 5; Nephrolepis, 6 ; Polypodium 
4. Second, the presence of chlorophyl in the epidermis is char- 
acteristic of a genus. Adiantum and Dryopteris possess chloro- 
phyl in epidermis and Nephrolepis and Polypodium do not. Third, 
the relative size and shape of the air spaces is constant through- 
out a genus. Fourth, the shape and arrangement of the cells are 
somewhat constant. Fifth, the presence of palisade tissue and the 
number of cells in its depth are constant through a genus. 
These characteristics taken one with the other appear to form 
a means sufficient to distinguish one genus from another. 
No, Cells No, Cells Depth Chlor. Air 
in in of ag Spaces 
Mesophyl Palisade Frond Epidermis 
PRES aot merrier meen neceie —— 
Adiantum 
Capillus-veneris 2 °o 106 Chlor. Large 
tenerum 2 ° 27 Chlor. Large 
pedatum 2 ° 70 Chlor. Large 
trifidum 2 o 77 Chlor. Large 
Nephrolepis ‘ 
molle 2 ° 110 Chlor. Small, very 
Phillipinensi 5 2 IIo Chlor. Small, 1 cell 
tuberosa 6 2 198 No Chlor. Small, x cell 
davalloides 6 2 185 No Chlor, Small, 1 cell 
Collengerii 6 2 176 No Chlor. Small, 1 cell 
exaltata 6 2 176 No Chlor. Small, 1-2 cells 
Polypodium 
Bereumi 4 2 176 No Chlor. Large, very 
vulgare 4 2 198° | No Chlor. Large 
Teptans 4 2 132 No Chlor. Large 
Dryopteris 
Mexicana 5 2 171 Chlor. Large, very 
Filix-mas x 2 146 Chlor. — Large, very 
Thelypteris 5 2 143 Chlor. Large, very 
triptera 5 2 114 Chlor. Large, very 
falcata 6 2 440 Chlor. Large, very 
Boranican LABORATORY, NORTHWESTERN UNIVERSITY, 
EVANSTON, ILL. 
