31 
Iir. STRUCTURAL ADAPTATIONS. 
The following oxpoBition of the adaptive nature of fern structures is 
arranged according to the anatomy and physiology of the plants, instead 
of according to outside conLlitions^ for two rt^'a^on.s. One of these is that 
every botanist is more familiar with" the outline? of plant anatomy and 
physiology than he is with the environment of any tropical plant and 
he therefore would always stand on unfamiliar ground if the other 
arrangement wore adopted; the other reason is that each part of the 
plant constitutes an environment for the rest of the plant — 1 liave already 
shown that largeness of frond usually demands thickness of epidermis- 
and tliat it would not be easy to fit correlations into a classification based 
on outside conditions. 
THE VEGETATIVE rUOND, 
Size. — It has already been shown that large fronds are characteristic 
of habitats having a moist air, and that, on the other hand, large fronds 
must by virtue of their size be more or less xerophytic in their finer 
structure. Very large fronds must have stout stipes and rhizomes which 
arc well anchored. The caudex of the huge rain forest variety of 
Aspidium leuzeanum is 10 centimeters in diameter. All our Dcnns- 
taedtias except D. erythrorachis have very stout, prostrate rhizomes. The 
enormous fronds of Angioptcris and Marattia spring from a globose 
caudex which often is 30 centimeters in diameter. Epiphytes have 
comparatively small fronds, the few exceptions being supported in an 
exceptional manner, namely, Asplenium musaefolium, Polypodium hera- 
cleum, and Platycerium, by massive nests which, in large specimens, 
coinpletely invest the supporting branch or trunk, as is sometimes the 
case with the stout rhizomes of Polypodium musaefolium, Drynaria qucr- 
cifolia and Thayeria. 
The margins of large fronds are always reinforced to give a protection 
against tearing. This protection may l)c by marginal anastomoses of the 
veins, as is the case in Syngramma, Callipteris cordifolia, Asplenium 
musaefolium and A. Phyllitidis; by more copious anastomoses in a gix-at 
many ferns, such as Drynaria and its relatives; by walls merely thicker 
near the margin, as in Cyclopeltis ; or by a more or less broad and rigid 
cartilaginous border, as is the case in Bymenolepis, Polypodium afflm, 
P. heracleum, Dryosiacliyum, TJiayeria, Phoiinoplcris and AcJirosticJiiun. 
When the margin is deflexed, a very common occurrence, it is less likely 
to tear. If the frond is lo])ed or incised, the sinuses are the places 
needing reinforcement. Tliey are reinforced by the venation of Goniop- 
terls and Callipteris and by a broader border of cartilage in Polypodium 
a/fine and other species. The special reinforcement of the sinuses can 
serve only as a protection against tearing, but the reinforcement of the 
margin as a whole is equally a protection against gnawing animals. It is 
