Development of the Fern Leaf G] 
is known of the various other ways in which it may be possible 
for conditions of environment to affect these leaves. 
In the cases of some species the extreme height of leaf- 
development seems to be attained under certain conditions only. 
These conditions, whether of environment or not, apparently are 
often such as tend to produce extreme luxuriance and fertility in 
the plant. It will readily be seen that this is not remarkable; 
that if a species’ leaf has a tendency to develop along certain 
lines, under conditions particularly favorable to the plant’s 
growth, development of the leaf might easily be carried along 
those lines farther than usual, or sometimes farther than ever 
before. Among our northeastern ferns we have apparently, in 
Polystichum acrostichoides, at least one authentic case of a species 
in which the plants, under certain conditions of environment, 
produce leaves portraying a height of leaf-development far be- 
yond the usual one, and lapse to their usual state when the con- 
ditions are altered in certain respects, producing then the usual 
mature leaves only. 
It is a well-known fact that conditions of environment some- 
times apparently induce changes in a species’ leaf. Conditions 
of environment acting upon the plant, during the period of the 
leaf-development, or even upon the individual leaves of its se- 
ries illustrative of this development, during their formation, can- 
not, however, be said to account, at least in the cases of certain 
species, for some of the changes that the species’ leaf undergoes 
in its development; for these changes are seen to be the same 
under conditions of environment obviously widely different. For 
instance, I have seen the leaf of Asplenium platyneuron undergo 
the same changes in the series of leaves borne by plants growing 
