in which specialization has gone farthest. 20 All the anticlinal walls of 
N. varius are freely collapsible. N. adnascms has hypodermal walls 
8 fi thick, but still, as they are placed, subject to a slight folding; while 
the anticlinal walls of the parenchyma are accordion-like (fig. 18). 
Sections of a leaf, 0.5 millimeter thick (weather very dry), widened 
to 0.83 millimeter as soon as cut, and to 1.49 millimeter in water; 
and the walls were still pleated. N. nummularicefolius has a single, 
uppermost layer with rigid walls, the remainder, hyaline and green, 
being collapsible. Sections of a frond, 0.56 millimeter thick, widened 
to 0.67 millimeter when cut and - to 0.86‘ 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. 
Hydathodes. — Niphobolus is likewise the only genus having trichome- 
hydathodes. 21 These hairs are different in form, 22 those of each of our 
species being characteristic, and our most xerophytic species, N. adnascens. 
being glabrescent; 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 Niphobolus 
leaves, this movement must be fairly active. When the outside of the 
leaf becomes dry, the outer cells of these trichomes 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 vacuum 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. 23 
Yery many ferns have the vein- tips hyaline, and, as a rule, the clear 
spots are hydathodes, clear because of the absence of air-containing spaces. 
Such hydathodes are found, among other ferns, on Meniscium, Arthrop- 
teris, Nephrolepis, Asplenium vulcanicum, A. tenerum, A. Belangeri, 
A. scandens, Ilymenolepis, Niphobolus Lingua, Polypodium Zippelii, P. 
20 Giesenhagen, 1. c.« 
a Giesenhagen : Famgattung Niphobolus. P. 44, and under each species. 
22 Using the word hydathode to include water-absorbing structures. 
23 Giesenhagen : Schwendener Festschrift (1899), p. 5. 
