1909] LEAVITT—HOMOEOSIS IN PLANTS 



35 









and ultimately fall off after several layers of brown tissue have been 

 developed on the scar surface-to-be, is now shown by the pinnules 

 of the Pierson fern. The pinnules of the latter are discovered to be 

 deciduous, and a minute investigation shows that deciduity is secured 

 by the development of an absciss-layer at the base of the pinnule, 



the formation of brown scar-tissue. Thus the translocation of 



with 



structures 



ascertained occurrence: these structures have passed at once and 

 unaltered from the bases of the pinnae to the bases of the pinnules. 

 The beautifully soft and luxuriant aspect of such ferns as the 



Wh 



Fig. 4.— Pinna of Whitman fern. 



many small fronds. In this last-named variety, and in some others, 

 as Pierson's ekgantissima and superbissima, homoeosis has gone one 

 step farther than in the Pierson, so that we find a thrice-compounded 

 ^ af (fig- 4), and the segments of the third order have the frondlike 

 character-^ven to the circinnate apical growth. 



we b° rCtUm t0 thC maUer ° f normal castin S of leaflets, with which 

 egan. The fact that in an instance now before us positive evi- 

 dence of the translocation of absciss-layers, etc., from one part of 



enc 1 t0 an ° ther has been secu red, strongly corroborates the infer- 

 ence already made on more speculative grounds, that leaflet-abjection 

 general is to be understood as an imitative or repetitive phenomenon. 



Whitman^ UasT^ **"* ° riginated in the greenhouse of H. H. Barrows & Son, 

 s i*>rt from th^p' 19 ° 4 ' * S * bud " s P ort » u P° n a runner of the Barrows fern (itself a 



e Person).— Letter from H. H. Barrows & Son to the writer. 



