45 
According to its neccssityj a hyaline liypodermis has been difrerentiated 
. in one or more genera of every tribe, as shown in the table (pages 12 to 
IG). This is usually found only beneath the upper epidermis^ but under- 
lies tlie nether as well in Davallia solida, As2)lenwm mnmefoUvm^ Vuhjpn- 
dium incurvatam, P. alhido-sqvnmaima and P. mudlforme, and incom- 
pletely in Dryosiachyum and Aclirostichiun.' Tliis tissue is found only 
in xoro2>hytes and the notion lias sometime liad vogue tliat its fnnction is 
to act as a water store. ^J'Jiat this is not in general Die case, 1 have 
pointed out elsewhere^" among the ferns the walls of the hypodermls 
are almost invariably so thick that any change in size or foriiij wliirh is 
necessary if they are to give up any water, is quite impossible. Thus, 
in Poly podium albidO'Sqiiamatum tlie hypoderriial walls arp 1:;^ fi thick, 
almost obliterating the lumen; Ilumala yaitnardiana has two layers of 
hyaline cells with walls 8 to 10 /x thick, and //. par-vida has two layers 
with walls 12 fi thick. Species with thin hypodermal wnlls and some 
with walls thick enough to seem rigid if plane, have the walls wavy or 
angular (fig. 15), as scon from the surface and tlicrcfore not collapsible 
under vertical pressure; or there are thickened intruding folds of the 
walls (fig. 16), such as brace the stomata of Medeola and other plants. 
Such walls are found in all species of Phymatodes, and its ofTshoots, 
Drynariaj otc.j which have any diifercntiatcd hypodermis, and in Acliros- 
tichum. They are also found in the uppermost parenchyma layer of 
Davallia pallida, Dennsiaedtia Williamsi and Polypodium suhaurlcula- 
tam. Beside a hyaline hypodermis with thick walls with the Phyrna- 
lodes contour, P. sinuosum has the uppermost layer of green mesophyll 
and, in less measure, the next two or three layers, provided with heavily 
thickened lines (tig. 17), perpendicular to the surface, to prevent 
collapse. Giesenhagen^** reports the same structures in Niplioholus slig- 
mosns, N. Gardneri and other species, and cites Poirault as authority for 
their occurrence in some other species of Polypodium. The rays of the 
stellate parenchyma of Ilumafa parvula usually have thickened walls and 
the fine, close veinlets of Drynnrin and its relatives, Polypodium liera- 
deiim^ Dryostaclium and Thayeria are all connected witli tlie epidermis 
by bands of sclerenchyma, inhibiting even an iiicipient collapse. 
On the other, hand, there are a very few species provided with an 
evidently available store of water. Thus, Polypodium caudiforme, with 
two layers of non collapsible cells under the upper epidermis, has one 
layer of collapsible cells ncit the nether one. Tlie walls of the green 
parenchjuia of Lo.voyrammc iridifolin, Anfrophynm rctirvlafum, and 
Polypodium accedens are somewliat collapsible with loss of water, but 
not greatly so. Tn this direction again it is Niplioholus, of all our ferns, 
"37us Journal (1900) 1: 25. 
"GiesfiiJiagen: tichivcndciier Festschrift^ (1891)), 6, 8, ]", 18, Vl. I. l\irn<jut- 
tunff XiphohoJus (1901), G7-79. 
