190 NATURK STUDY. 
edge of the leaf is deftly turned over them, like a hem. In 
Pteris this hem is continuous, in Adiantum it is in sections, 
in Pellsea it is continuous, in all it is more or less modified 
and membranous. It will be interesting for the student to 
watch, from week to week, the growth of fruitful plants of 
the common brake. It may surprise him at first to note 
how small a proportion of the plants bear fruit. He may 
turn over hundreds of leaves where the plants are growing 
in shade, and find them all sterile. In sunny spots he is 
more likely to find the object of his search. Then, as the 
season progresses, he can watch the gradual swelling of 
the sporangia under the long, narrow indusium, until at 
length the pressure from within ruptures it along the inner 
edge and the fruit is exposed. 
There remain eight genera in which there is a true in¬ 
dusium. In them it consists of a scale-like outgrowth 
from the lower side of the leaf. Various forms are as¬ 
sumed by this special adjunct, according to the arrange¬ 
ment of the sori. The method of attachment, the texture, 
the condition of the surface and the evanescence or persis¬ 
tence of the indusia constitute systematic characters of 
great value in the identification of genera and species. 
In Onoclea the indusium is membranous and hood-shaped 
and is attached underneath the sorus. In Woodsia it is al¬ 
so attached underneath, like the cloth covering of a button. 
In most of the species it subtends rather than covers the 
sorus and is early reduced to a few thread-like segments. 
In Woodsia obtusa, however, it at first encloses the spor¬ 
angia and at length splits into a few rather wide, jagged 
lobes. In Dicksonia it is in the shape of a cup, open at the 
top and attached on the outer side to a toothlet of the leaf. 
For the indusia of Cystopteris search must be made early 
in the season, for the double reason that the plants fruit 
early and that the indusia very soon wither, leaving the 
sporangia naked. In the genus Dryopteris (Aspidium) 
