11 
fern, environments would be classified almost solely with regard to the 
physiological dryness of the air. In a general way, a classification can 
be made along this line, as this one is; but in detail it is not geograph- 
ically feasible, because of great and very local differences. As an 
illustration of these; the Negritos of Bataan dig holes more than a meter 
deep and hardly 30 centimeters across, to gather a wild yam ; these holes 
are common in the parang but harbor a fern vegetation characteristic of 
rocky gorges in the high forest. 
Compared with the moisture, the temperature plays a very minor role 
in the. control of the distribution of these ferns, and probably none at 
all in the modification of their structures, except indirectly as it influences 
their transpiration. Compared with the moisture of the air, that of the 
ground is likewise an insignificant factor both in distribution and struc- 
ture. This is more true of plants in general than many ecologists seem 
to realize, but it is, of course, especially so of a group like the ferns, 
a large proportion of which display their independence of ground-moisture 
by being epiphytes; moreover, poor water-absorbing or water-conducting 
organs increase a plant’s relative dependence on atmospheric moisture; 
ferns have ill-developed conducting systems, as compared with flowering 
plants. Tree ferns grow only where the air is most moist, in canons, or 
forested pockets and cirque-like basins, and the few scandent Polypo- 
diacece, in other than the wettest placed, are strongly xerophytic in 
structure. 
The moisture of the atmosphere and that of the soil are of course 
intimately related; if the ground is wet, air standing over it becomes so, 
and if the air in a given place has a very drying action on plants it will 
have the same effect on the soil. However, the influence of the moisture 
of the soil and of that of the air can sometimes very readily be distin- 
guished; for instance, in the cases of plants of different height, growing 
together on the same ground, it is wholly dependent on the atmosphere 
whether or not the taller plant must differ from the shorter. There is 
no environment so damp but that on the whole the taller ferns have 
more xerophytic leaf-structures than those the leaves of which are close 
to the ground ; but the limit of height is very different in different environ- 
ments in which the structures of low-growing plants are essentially alike. 
The soil in the low jungle never dries ; but there are times when the air 
does so, sufficiently and for a long enough period of time to make it an 
impossible habitat for any of the giant ferns of the rain forest or of any 
ferns of like structure and stature. 
In the following table are listed the ferns of the several habitats the 
structure of which I was able to study with fresh material ; the thickness 
of frond and of outer wall of each epidermis, the presence or absence of 
chlorophyll in the epidermis, and of a chlorophyll-less hypodermis, the 
number of stomata per square millimeter, and the average length and 
width of a stoma are givpn. 
