45 
According to its necessity, a hyaline hypodermis has been differentiated 
in one or more genera of every tribe, as shown in the table (pages 12 to 
16) . This is usually found only beneath the upper epidermis, hut under- 
lies the nether as well in Davallia solida, Asplenium musaefolium, Polypo- 
dium incurvatum, P. albido-squamcitum and P. caudiforme, and incom- 
pletely in Dryostacliyum and A chroslichum. This tissue is found only 
in xerophytes and the notion has sometime had vogue that its function is 
to act as a water store. That this is not in general the case, I have 
pointed out elsewhere 18 among the ferns the walls of the hypodermis 
are almost invariably so thick that any change in size or form, which is 
necessary if they are to give up any water, is quite impossible. Thus, 
in Polypodium albido-squamatum the hypodermal walls are 12 y thick, 
almost obliterating the lumen; Ilumata gaimardiana has two layers of 
hyaline cells with walls 8 to 10 y thick, and IT. parvula has two layers 
with walls 12 y thick. Species with thin hypodermal walls and some 
with walls thick enough to seem rigid if plane, have the walls wavy or 
angular (fig. 15), as seen from the surface and therefore 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 offshoots, 
Drynaria , etc., which have any differentiated hypodermis, and in Achros- 
tichum. They are also found in the uppermost parenchyma layer of 
Davallia pallida, Dennstaedtia Williamsi and Polypodium subauricula- 
tum. Beside a hyaline hypodermis with thick walls with the Phyma- 
todes contour, P. sinuosum has the uppermost layer of green mesopliyll 
and, in less measure, the next two or three layers, provided with heavily 
thickened lines (fig. 17), perpendicular to the surface, to prevent 
collapse. Giesenhagen 19 reports the same structures in Niphobolu-s stig- 
mosiis, 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 Ilumata parvula usually have thickened walls and 
the fine,' close veinlets of Drynaria and its relatives, Polypodium hera- 
cleum, Dryostaclium and Tliayeria are all connected with the epidermis 
by bands of sclerenchyma, inhibiting even an incipient 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 noncollapsible cells under the upper epidermis, has one 
layer of collapsible cells next the nether one. The walls of the green 
parenchyma of Loxogramme iridifolia , Antrophyum reticulatum, and 
Polypodium accedens are somewhat collapsible with loss of water, but 
not greatly so. In this direction again it is Nipliobolus, of all our ferns, 
ls This Journal (1906) 1 : 25. 
10 Giesenhagen : Schwendener Festschrift (1899), 6, 8, 17, 18, PL /. F.arngat- 
tung Niphobolus (1901), 67-79. 
