18 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. 
