Development of the Fern Leaf 7 



is known of the various other ways in which it may be possible 

 for conditions of environment to afifect 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, undef 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 



