434 
depth of the mesophyl is from four to six, except in P. cretica 
which has ten. 
Scolopendrium has no palisade tissue; has chlorophy! in the 
epidermis ; has many large air spaces and irregularly shaped cells; 
and nine layers of cells in the mesophyl. 
Sachs says, “ According to Stahl the palisade cells constitute 
that form of assimilating tissue, which is especially produced by 
intense light striking the leaf directly, and leaves grown in the 
shade produce chiefly or only spongy parenchyma.” In the ferns 
I have examined the form of the cells of the mesophyl does not 
seem to depend on the intensity of the light, as most ferns grow 
naturally in diffuse light and the cultivated species which I have 
sed were grown in the shade. In some of these ferns a marked 
palisade structure is apparent as in Dryopteris falcata where it 
consists of two layers of cells, of rectangular section; Pteris 
aquilina has also two layers; Pteris sagittifolia has one \ayer ; Pteris 
Cretica, three and Blechnum serrulatum,two. Intermediate between 
this evident palisade structure and none at all, I find some which 
have one or more layers of closely placed cylindrical cells, with 
axes nearly equal. Polypodium aureum has two layers of these 
cells. LPolypodium vulgare, grown in bright sunlight, and Nephro- 
lepis exaltata have also two layers. Dryopteris Mexicana, D. Filix- 
mas, Asplenium fabianum, Pteris serrulata, and all the five species 
of Adiantum which I examined have no palisade parenchyma. 
The palisade tissue could not have been formed by intense sun- 
light, but rather it seems that the presence or absence of palisade 
parenchyma is very nearly constant throughout each genus, that 
is, it is a generic characteristic. ; 
The ferns which have no palisade cells possess chlorophyl in the 
epidermis. It may be that this chlorophyl in the upper epider- 
mis takes the place of a palisade layer. Terlitaki in a paper on 
Struthiopteris and Preris says that ferns are distinguished by the 
presence of chlorophyl in the epidermis, Many of the ferns which 
which I examined jhad chlorophyl in the epidermis layers, as the 
genus Dryopteris, which had also a palisade tissue, and four spe- 
cies of Pteris, Blechnum serrulatum, Scolopendium and Asplenium 
There are, howeve?, many which have no chlorophyl in the epi- 
dermis, as the genus Nephrolepis, Polvpodi 
. Dieigoe These have without exception a palisade structure. 
tum and four species of 
