46 
in Avhich specialization 1ms gono iarthest.'* All the anticlinal walU of 
N, varins are fruoly collapsible. N. adnasrcns has hypodennal walls 
8 fx thill-, hut stilh as thov are placed, subject to a slight fohling; while 
the anticlinal Avails of the parenchyma are accordioii-like (fig. 18). 
Sections of a leaf, 0.5 millimoter thick (weather very dry), widened 
to 0.83 millimeter as soon as cut> and to 1.49 milliinetor in water; 
and the walls were still pleated. N. nummnlanfvfoVuts has' a single, 
uppermost, layer with rigid walls, the remainder, hyaline and green, 
being collapsible. Sections of a frond, 0.5G millimeter thick, widened 
to 0.G7 millimeter when cut and to 0.8G millimeter when wet. N, 
lingua has 2 to 3 layers of hyaline cells, of which only the inmost can 
collapse at all, and this much less readily than can the parenchyma. 
Ilydathodes. — l^ipliobolus is likewise the only genus having trieliome- 
iivdatliodes.^^ These hairs are different in form,^^ those of each of our 
species being characteristic, and our most xerophytie species, N, adnascens, 
being glabreseent; but they are all alike in insertion, each hair growing 
in a pit which is practically filled by the basal cell of the trichome. 
This basal cell is alive, with considerable evident contents. When the 
leaf is damp, this contents fills the cell; it can then absorb water 
from the cells borne on it whether they are dead or alive, and give 
water to the cells within. Judging by the high turgor in Niphohohis 
leaves, this movement must be fairlv active. When the outside of the 
leaf becomes dry, the outer cells of these trichoiucs lose their water 
and promptly draw on the basal cell. If the connection were main- 
tained, the basal cell would then supply itself from the interior of the 
leaf. But this does not happen because its protoplasm instead of 
keeping in connection with the cells within and without, shrinks away 
from its wall and contracts into a lump touching but one end of the cell. 
A dead air space, or approximate vacvmm in the basal cell then protects 
the interior of the leaf from evaporation. The protoplasm of the 
basal cell collapses instead of maintaining its turgidity, because it 
loses water outward without faster than it can get it from within, this 
condition must be due both to the very high turgor of the mesophyll and 
to the unequal permeability of the end-walls of the basal cell, their 
outer end being pitted.^^ 
Very many ferns have the vein-tips hyaline, and, as a rule, the clear 
spots are hydathodes, clear because of the ah^ence of air-containing spaces. 
Such hydathodes are found, among other ferns, on Menisciuni, Arthrop- 
ieris, Nephrolepis, Asplenium vulcanicnm, A. tenerum, A. Belangeri, 
A. scandcns, IlymenoJepis, 'Niplioholus Lingua, Pohjpodium Zippelii, P. 
r 
" Giesonliagt'ii, 1. c. 
^Giespiihrtjren: Fanigaliwu} N^phohohifi. V. 44, luul iititlei* each >>po(-ies. 
"Using the word hyaathuae to inchidt' WiiU'i-ahhuibinjr structuivs. 
=^Ciesenhagen: Schivoidener Festschrift (1809), p. 5. 
