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 



