31 
III. STRUCTURAL ADAPTATIONS. 
The following exposition of the adaptive nature of fern structures is 
arranged according to the anatomy and physiology of the plants, instead 
of according to outside conditions, for two reasons. One of these is that 
every botanist is more familiar with the outlines 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 were adopted; the other reason is that each part of the 
plant constitutes an environment for the rest of the plant — I have already 
shown that largeness of frond usually demands thickness of epidermis — 
and that it would not be easy to fit correlations into a classification based 
on outside conditions. 
THE VEGETATIVE FROND. 
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 
are well anchored. The caudex of the huge rain forest variety of 
Aspidium leuzeanum is 10 centimeters in diameter. All our Denns- 
taedtias except D. erythroradiis have very stout, prostrate rhizomes. The 
enormous fronds of Angiopteris 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 Ptatycerium, by massive nests which, in large specimens, 
completely invest the supporting branch or trunk, as is sometimes the 
case with the stout rhizomes of Polypodium musaefolium, Drynaria quer- 
cifolia and Thayeria. 
The margins of large fronds are always reinforced to give a protection 
against tearing. This protection may be 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 great 
many ferns, such as Drynaria and its relatives; by walls merely thicker 
near the margin, as in Gyclopeltis ; or by a more or less broad and rigid 
cartilaginous border, as is the case in Hymenolepis , Polypodium affine , 
P. heracleum, Dryostachyum, Thayeria, Photinopteris and Achrostichum. 
When the margin is deflexed^a very common occurrence, it is less likely 
to tear. If the frond is lobed or incised, the sinuses are the places 
needing reinforcement. They are reinforced by the venation of Goniop- 
t»ris and Callipteris and by a broader border of cartilage in Polypodium 
affine 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 
