1 8 Development of the Fern Leaf 



formation of each successive segment of each series is not begun 

 until the segment preceding it on the same side of the midvein 

 has become distinct. It is often absent when formation of the 

 segments of one or more succeeding series is begun upon segments 

 of one series while the latter segments are still in an incipient 

 state. Instances of the latter kind are often seen in very young 

 leaves: in such cases the effect of clusters of lobes is often pro- 

 duced. An analogous and particularly common case is the begin- 

 ning of the formation of segments on a leaf -blade when the forma- 

 tion of the leaf-blade itself has barely begun. 



From the above account some idea can be formed of the 

 diversified results that may be produced in a leaf with pinnate 

 free venation, by the simple occurrence of incisions between the 

 primary branches of the leaf's midveins, with or without enlarge- 

 ment of the segments so formed and with or without development 

 of a midvein from the primary branch, as a base, contained 

 within each segment. 



Anastomose pinnate venation differs from free pinnate vena- 

 tion in that the midveins bear, instead of free branches, branches 

 that unite with one another, usually by means of their veinlets. 

 Between the two kinds of venation all gradations exist, as the 

 result of some veins uniting and others of the same leaf remain- 

 ing free. The occurrence of an occasional areole in a leaf whose 

 venation, as a rule, is wholly free is common 



It is a curious fact that in all four of our northeastern species* 

 whose leaves, when mature, possess pinnate venation more or 

 less conspicuously anastomose, the venation in the first stages of 

 the leaf is free. 



*Camptosorus rhizophyllus, Anchistea Virginica, Lorinseria areolata, and Onoclea 

 sensibilis. 



