Mature: study. 
170 
absence of a special covering of the sori called the “indus- 
ium,” and the different shapes assumed by that curious 
adjunct. The arrangement of the species is mostly depend- . 
ent on characters- more evident to the ordinary observer, 
such as the shape of the leaves, the disposition of the veins, 
and so on. 
One of the first points which is likely to attract the atten¬ 
tion of the amateur botanist is the unlikeness of the sterile 
and fertile leaves in some species. The ferns are in such 
cases said to be “dimorphous.” The differences range in 
degree from slight to total, and would appear to be the re¬ 
sult of the different proportions between the vegetative and 
reproductive energies in different genera or species. Where 
the reproductive -energy is in excess the difference should 
be greater, and where the vegetative energy is in excess it 
should be less. If this is true, the greatest difference be¬ 
tween the sterile and fertile leaves should be found in those 
members of the family which have been least differentiated 
from the primitive type. Specialization tends to promote 
the vegetative and reduce the reproductive energy. In the 
long run the principle holds that the lower a plant is in 
the evolutionary scale the greater, in proportion is its re¬ 
productive energy. A mushroom, the lowest of all famil¬ 
iar plants in organization, is practically all fructification. 
At the other extreme stand the seed-bearing plants,and in 
them the reproductive functions are entirely subordinated 
to those of vegetation. 
In the fern family this principle undoubtedly prevails. 
While in our local species the illustrations of it are not 
numerous there appears to be an example in the very gen¬ 
us which heads the list and which, for sufficient reasons, is 
considered to have undergone the least specialization. 
This is the genus Onoclea, comprising only three known 
species, of which our area contains two species, the sensi¬ 
tive fern, (O. sensibilis), and the ostrich fern, (O. struthi- 
