34 
This is equally the case when the margin of an otherwise essentially 
plane frond or division is deflexed, as in N ephrodium 1712, N. interme- 
dium, Oleandra nitidki, Microlepia pinnata, Dennstaedtia Williamsi , 
Callipteris cordifolia, Antrophyum plantagineum , A. reticulatum , Poly- 
podium punctatum, P. 1741, P. incurvatum, P. palmatum, Photinopteris 
and Achrostichum. In the majority of these plants the margin is sharp 
as well as deflexed, but that of P. incurvatum is rather thicker than the 
rest of the frond, as a result of increase in the sclerenchyma. A few 
species, namely, Loxogramme conferta, Scolopendrium schizocarpum, 
Antrophyum latifolium. Polypodium ccespitosum, P. dolichopterum and 
Thayeria, have a sharp margin which 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 move in drops, but 
might move in a film around even a rather sharp edge ; but so long as 
the nether 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 
Polypodium Schneideri and P. macrophyllum. 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 ciliate margin is also, as a rule, an obstacle to the passage of water ; 
for, if the hairs are 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 nether 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 N ephrodium pi'ocurrens, N. 1685, Oleandra colu- 
brina nitida, Polypodium celebicum and Dryostachyum pilosum. Several 
Philippine species of Elaphoglossum are remarkable in this respect. 
If 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 vewy 
important factor. Unless 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 which gravity can act to remove the drop from the leaf, it will 
inevitably assume a spherical form and fall. Of course,. if the contact is 
sufficiently reduced, the drop will become spherical independently of grav- 
