Development of the Fern Leaf 5 
stage only of normal development, for this stage, as will be seen 
farther on, is apparently attained, in the cases of at least some 
species, under particularly favorable conditions only, so that the 
leaves of many plants of a species may never portray it, that 
nevertheless reach a fair size, compared with the more highly 
developed leaves, and bear sori freely. For practical purposes 
we may, however, regard as mature both those leaves that have 
attained the latter state of development and those more highly 
developed. 
The species’ leaf on becoming fertile often becomes modified 
in character. The change is sometimes very great, as in Onoclea 
sensibilis. It is often slight. When it occurs, often only those 
leaves that portray the two extremes of the transition are to be 
seen on some plants, or on those plants at certain times, but in 
such cases it sometimes, if not always, happens that leaves portray- 
ing the intermediate stages are to be seen either on the same 
plants at other times, or on other plants. A series of leaves illus- 
trative of the transition can thus be collected in the same manner 
that a series of leaves illustrative of the leaf’s development can be 
collected. 
- The degree of development and the size of each successive 
leaf of the series illustrative of leaf-development borne by the 
individual plant seem to be governed to some extent by the 
plant’s degree of vigor. The leaves of the vigorous plant are apt 
to be considerably larger than the leaves of the weaker, even 
than such as happen to portray the same stages of development. 
In addition, if the plant is vigorous, the discrepancy between the 
degrees of development shown by any two consecutive leaves of 
the series is apt to be greater,so that the series consists of fewer 
