1gtt] SHREVE—JAMAICAN HYMENOPHYLLACEAE 199 
method, the writer’s experiment with a single leaf of Trichomanes 
is comparable with those of Taopay and SyKEs with severed shoots 
of Potamogeton. In spite of the violence which is done to the normal 
functioning of the leaves of flowering plants by removal from the 
plant (which will be shown below not to hold true of the filmy 
ferns), the writer is inclined, on the concordant evidence of the 
potometer experiments described, to believe that there is a stand- 
still in the transpiration stream of a filmy fern when the leaves 
are wet. In exact analysis it is unthinkable that there should not 
be a simultaneous loss and intake of water through the leaf sur- 
face, but the backward movement of the potometer bubble shows 
that the absorbing activity is greater than the exudation when 
leaf-absorption is given play through the removal of the roots. 
In the intact plant, however, leaf-absorption is devoted solely to 
the maintenance of leaf turgidity, and the movement of root- 
absorbed water toward the leaves is in abeyance. When in its 
natural habitat, the filmy fern is now under aquatic conditions, 
now under those of a land plant. There may or may not be a 
transpiration stream in a plant according to the state of wetness 
of its leaves, and when the leaves are dry the ensuing root-absorp- 
tion will or will not be able to meet the demands of water loss in 
the leaves according to the species and the humidity conditions 
in which it is placed. From the evidence given it appears that in 
hature there is now a slight transpiration stream in a given plant 
and now an intake of water by the leaves, accompanied by a cessa- 
ton of conduction in the vessels. 
ToraL suBMERGENCE.—A completely wet filmy fern is in a 
State equivalent to that of a submerged aquatic, excepting that it 
1S under much more favorable conditions as respects the aeration 
of the water surrounding it. In order to determine what would 
be the effect of the stoppage of the transpiration stream for a con- 
siderable period, a number of plants were grown in total submer- 
Sence, and in order to make these conditions as natural as possible 
the water was changed every four days and was aerated twice 
daily by injecting a fine stream of water with a large pipette. 
Flourishing individuals of the following species were selected, 
with their roots intact: Trichomanes rigidum, T. radicans, T. 
