TRANSFORMATION OF SPOROPHYLLARY. 173 



tant from, the mid-vein, and arches slightly outward, or away 

 from the mid-vein, partially covering the group of sporangia. 



The first step in the transformation of the sporophyll is the 

 lateral spreading of the pinnae, so that they stand out more or 

 less strongly in a plane corresponding to that of the vegetative 

 leaf. At the same time there occurs a partial unfolding of the 

 revolute pinnule. This occurs by an increase in the number 

 of cells of the lamina, and a corresponding decrease in the 

 growth of the cells which go to form the sori. The marginal 

 cells of the lamina increase more rapidly than those toward 

 the mid-vein and the base. A few of the terminal pinnae in the 

 normal sporophyll bear no pinnules, but are very much like 

 the pinnules of the lower pinnae. The simplest condition of the 

 unfolding of the revolute pinnae or pinnules is the partial 

 expansion of these terminal ones, more advanced stages pro- 

 ceeding along down from the terminal portions of the pinnules. 



The grades of transition are correlated with the circinate 

 development of the leaf, the later developed portions showing 

 a more highly developed vegetative expansion and function 

 than the early developed portions of the same leaf. The pinnae 

 then, in the simplest and intermediate stages, have a more 

 or less clavate or spathulate outline. The pinnules in an indi- 

 vidual pinna may vary from quite strongly revolute ones to 

 those which are fully expanded and nearly plane. 



Occasionally transformed fertile leaves are found which show 

 a tendency to an abortive development, i.e. they remain quite 

 small, the stipe rather short, the pinnae and pinnules very 

 short, though they may be quite fully expanded. 



Comparing individual sporophylls, all grades of transition are 

 present from the completely differentiated fertile leaf to the 

 sterile one. In several cases it was impossible to determine 

 with certainty to which phase of the dimorphism the leaf pri- 

 marily belonged, or for which it was originally intended. Some 

 of these leaves which were at first doubtful were finally found 

 to possess rudimentary indusia on a few of the lower pinnae, 

 these occurring on the basal pinnules. 



The fully or partially expanded pinnules, when the transfor- 

 mation has not been carried too far, are more or less obtuse. 



