Development of the Fern Leaf 3 
the young plant as it arises from the prothallus, portray succes- 
sively the successive changes that the species’ leaf undergoes in 
its development. After the mature stages of leaf-development 
are reached, leaves continue to form on the plant, but, at least as 
a rule, portray only the mature phases of the species’ leaf. 
The degrees of development shown by the individual leaves 
of the series illustrative of leaf-development vary in different 
plants of the same species. The degree shown by any one leaf 
of the series borne by any one plant is commonly more or less 
greater than that shown by any one of the leaves preceding this 
leaf in the series, and more or less smaller than that shown by 
any one of those following, but bears no other relation to the de- 
gree shown by any leaf of the series. The discrepancy between 
the degrees shown by any two consecutive leaves of any series 
may be minute or larger or even great. The number of leaves 
composing the series of any one plant of a species is thus not 
necessarily the same as the number composing the series borne 
by any other plant of that species. 
The development of the species’ leaf as a whole commonly 
advances steadily, but the development of its parts often fluctu- 
ates. For example, given two consecutive leaves of the series of 
leaves illustrative of leaf-development, the degree of development 
of the second leaf as a whole is commonly greater than that of 
the first, but the degree of development of one of its parts is often 
less than that of the corresponding part of the first, while the de- 
gree of development of the corresponding part in the leaf next 
succeeding these two leaves in the series may or may not be 
greater than that of the corresponding part in either of the two. 
Even after the mature stages of leaf-development have been 
