60 



THE FERN BULLETIN 



ally belong to the outer circle, as befits the group that 

 is to develop first. The sensitive and ostrich ferns are 

 other species in which the zones of fronds are very dis- 

 tinct. So pronounced is this, and so far has each kind 

 developed before unfolding, that each is usually incap- 

 able of taking up the functions of the other in cases 

 where the destruction of one kind makes such exchange 

 necessary or desirable. From efforts on the part of the 

 plant to supply vegetative tissue to leaves designed 

 originally for. spore-bearing, only, we owe the various 

 "obtusilobata" forms occasionally reported. The dif- 

 ferences in zonation here mentioned are most pro- 

 nounced in ferns with dimorphic fronds, but evidences 

 of the same thing, more or less distinct may be found 

 even in those ferns that have the fertile and sterile 

 fronds essentially alike in outline. As a usual thing, 

 the spore-bearing leaves are produced after the vege- 

 tative leaves have unfolded and when we find a plant in 

 full fruit in late summer, that lacked spores in spring, 

 it is due to the developing of the fertile leaves later. 

 This is especially true and most noticeable in ferns that 

 produce their fronds in crowns, but even in those 

 species with running rootstocks, we commonly find 

 evidences of zonation. Following out the idea of zona- 

 tion we find among many of the fern allies that not only 

 are the sporophylls assembled in zones but the zones 

 terminate the central axis or branch. Under such cir- 

 cumstances the shoot begins to take on many of the 

 characteristics of the flower and if we allow the defini- 

 tion of a flower as a shoot beset with sporophylls, it 

 really is a flower. In the plants in which the flower 

 comes to its highest development this structure is es- 

 sentially a group of two kinds of sporophylls set 

 round with sterile leaves called petals and sepals. Did 

 ferns, instead of selaginellas, produce two kinds of 

 sporophylls, the whole fern plant with its crown of 

 fronds, would be very like a flower. 



