CHAPTER I 



DEVELOPMENT OF THE FERN LEAF 



AMONG the reasons for which study of the development of 

 form and venation in fern leaves appears desirable, the following 

 are conspicuous. 



In many cases, leaf characters supposedly specific are features 

 of only comparatively late stages of development of the species' 

 leaf. As might be expected from that fact, this development 

 sometimes, by exceeding its usual limits, partly or wholly ob- 

 literates such characters. In any delimitation of fern species, it 

 is thus necessary to take into consideration the leaf-development 

 of each species. In order to do this, and sometimes .also in order 

 to distinguish between what is due to this development and what 

 is due to subspecific or other variation, it is necessary to know 

 something of the modes of development of the form and venation 

 of the leaf of each species in question. Instances are not wanting 

 in which failure to understand the phenomena of such develop- 

 ment has misled fern students into interpreting different stages 

 of one species' leaf as leaves of different species or of different 

 varieties of the same species. 



It is evident that in many cases a clearer conception of the 

 genetic affinities of fern species can be formed with knowledge of 



