34 
-Tliiti U uqiially the case when the margin of an otherwise essentially 
plane frond or division is deflexed, as in X e ithro<rut m 1712, N, interme- 
dium, Ohandra nituh; MkroUinn pinnata, Dcnndacdtia Willuimsi, 
CalUpteris' cordlfolia. Antra pit ij urn pJaniaijincum, A. rcticahifum, Poly- 
podium pundaium, P. 1741, P. incurrafum, P. pahnaium,, Phofivnpfm's 
uiul Achro:<(ichinu. In the niajoritv of these plants the margin is sharp 
as well as doiiexed, bnt that of P. incurvaium is rather thicker than the 
rest of the frond, as a result of increaiie in tlie scleronchyuia. A few 
species, namely, Loxogramme confcrta^ Scolopen'driuni scliizorarpum, 
Antrophyum Jatifolium, Pbhjpodium ccpspxtofium , P. doUchopierum and 
Tliaijeria, have a sharp margin wliich is not deflexed. The sharpness 
alone must prevent a drop of water from running to the nether surface. 
If the entire surface is wet, water need perhaps not uio\e in drops, hut 
might move in a fihn around even a rather sharp Q^gO] hut so long as 
the riether surface is not wet, or is imperfectly so, the surface tension 
of a drop would cause it to become spherical on an edge as sharp as the 
ones under discussion, and it would therefore fall off. 
Over-fullness of the margin causes an effect like' that produced by 
convexity of frond. Such margins are wavy or crisped, alternately raised 
and deflexed. Water will, of course, run to the margin where it is 
lowest and only to this point; these are the places from which it would 
have to run upward if it were to wet the nether surface. Examples are 
PoJypodmm Schneiden and P. macro phyJIum. Such fronds can hardly 
be torn because the extra length of their margins allows them merely to 
straighten if the fronds are bent toward the other side. 
A ciliatc margin is also, as a rule, an obstacle to the passage of water; 
for, if the hairs arc not wet, a drop must pass over their ends from which 
it will inevitably fall off, but if they are wet, they usually furnish an 
opportunity for water to run down far enough to fall instead of allowing 
it to pass to the netlier surface. As a matter of fact, neither the hairs 
nor the cuticles of plants in general are very- readily wet. Local ferns * 
with ciliate margin are Nephrodium procurrens, N, 1085, Ohandra colu- 
hrina nitida, Pohjpodmm celehicum and Dryostachyum pilosum. Several 
Philippine species of EJaplioglossnm are remarkable in this respect. 
It we suppose a frond to be horizontal, then the possibility of a drop 
passing from the upper to the nether surface depends on the area of 
contact which it can preserve with the frond in rounding the margin. 
If the frond is in some other position, the area of contact is still a very 
important factor. I^nless this area is sufficient to allow the drop to 
flatten into a broad enough oval markedly to reduce its relative surface 
over what it would be were the drop a sphere, and thus to overcome the 
force with whieh gravity can act to remove the drop froui the leaf, it will 
inevital>ly assume a spherical form and fall. Of course, if the contact is 
sufficiently reduced, the drop will l)ecome spherical independently of grav- 
