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dried out by reason of their limited absorption area or lack of vascular system 
or both. It seems probable that the large papillae found on the surface of the 
cells of many species may be but an added protection against dessication, e. 
g. Orthotrichum, Andreaea, Thuidium , Anomodon and Thelia. The fact 
that such species hs Anomodon rostratus and Thuidium delicatulum grow in 
moist situations does not outweigh the fact that these genera as a whole are 
xerophytic. The papillae}formed by the projecting angles of the leaf-cells as 
in Bryhnia Kaurin belong in a wholly different category. We have a few 
cases of xerophyte species with papillose leaf-cells in genera which do not as 
a rule have papillae, e. g, Dicranum spurium and D. condensatum. 
While considering this topic it is well to call to mind the fact that many 
mosses that are drenched with moisture much of the time are at others exposed 
to drying winds of high velocity and great absorbing power. Plants grow- 
ing on exposed rocks in mountains furnish a good illustration of this. 
Andreaea petrophila grows freely all over Mt. Mansfield, Vermont, at an 
altitude of 4000 feet and over, but fruits freely in sheltered spots only. The 
evidences of the importance of water supply and retention in the mosses are 
so abundant and conclusive that further enumeration is not needed here. It 
is entirely probable that the failure of any terrestrial plants below the vas- 
cular cryptogams to attain commanding size is due to the lack of ability to 
supply and retain sufficient moisture for a greater development. 
All careful students of our mosses have noted the large number of quad- 
rate, thick- walled alar cells found in pleurocarpous mosses growing on the 
bark of trees and similar xerophytic habitats even in genera or families in 
which such cells are not usually numerous, e. g. Pylaisia , Eurhynchium 
myosuroides Schimp., Isothecium , Rhytidium rugosian (L.) Kindb. Also 
note that Pylaisia Schimperi (Hedw. ) Card, growing in dry exposed situa- 
tions such as old apple trees has a much larger number of these cells than P. 
intricata (Hedw.) Cardot (P. velutina of authors) which grows in moist 
woods. 
On the other hand the alar cells of aquatic or subaquatic pleurocarpous 
species tend to become enlarged and inflated, e. g. Drepanocladus (Harpi- 
dium), Scleropodium obtusifolium , Campy lium stellatum , Brachythecium 
rivulare , etc. In these examples it will be noted that the other leaf-cells 
are mostly exceedingly long and narrow, making the contrast much more 
striking. The meaning of this modification is very obscure. The almost uni- 
versal differentiation of alar and basal cells in mosses is possibly due to the 
fact that their part in photosynthesis is very small and they are the ones most 
free to respond to other than light relations. 
It may possibly be that these thin walled and inflated alar cells of 
aquatic mosses are structures facilitating osmosis between the contents of 
the cells and the water outside. It is also possible that scarious leaf bases 
like those of Poly trichum may be water absorbing organs. It is pretty gen- 
erally admitted that the large thin-walled cells of the leaves of Sphagnum 
serve to absorb and retain water for by means of these cells the plant is 
almost sponge-like in its power to absorb and retain moisture. 
