4 Development of the Fern Leaf 
reached, while two leaves of the same plant may as wholes agree 
approximately in point of development, the degrees of develop- 
ment of similar parts in these leaves may or may not be uniform: 
but this is almost equivalent to saying merely that no two leaves 
are likely to be exactly alike. 
It is evident that under the above conditions, if we want a 
complete series of leaves illustrative of the gradations of change 
that a species’ leaf undergoes in its development, we shall proba- 
bly not be able to find it in the series of leaves borne by any one 
plant, but we may be able to select the leaves necessary to form 
it from the series borne by different plants of the species. 
It is also evident that, while there is apparently no certainty 
that any one of the leaves of the series borne by any one plant 
shall match in points of development any leaf of the series borne 
by any other plant, instances may occur in which one does. It will 
also be seen that instances may occur in which leaves portraying 
certain stages of development usually or always appear, under 
normal conditions, in the series borne by each plant of a species, 
however the leaves preceding or following them may vary in 
degree of development in the cases of the different plants. I 
have found, on examining a large number of young plants of 
each of certain species, taken from different localities, that nearly 
all the first leaves produced by the plants of each of these species 
portrayed the same stage of development. 
It is often difficult to say at what stage of development the 
species’ leaf becomes mature. We can scarcely say at the stage 
when it first bears sori, for in some species this stage appears to 
be different with different plants, and in some the leaf bears sori 
at an extremely early stage. Nor can we say at the ultimate 
