Purple Cliff-Brake 59 



incipient basal primary pinnae. In Fig. 6 they are shown fully 

 separated from the part of the blade above them. In later leaves 

 additional pairs of primary pinnae are formed, and in time a 

 series of secondary pinnae (Pis. IX, X). 



Each pinna becomes distinct from the part of the leaf beyond 

 it before its subdivision into segments, if occurring, is begun, and 

 nearly always before formation of any pinna beyond it is begun: 

 before or during the formation of the incision that renders the 

 pinna distinct from the part of the leaf beyond it, the part of the 

 leaf that is to form the pinna usually elongates in the direction that 

 the pinna's apex is to take. As a result, at any stage of the leaf's 

 development each of the leaf's ultimate divisions or apical sections, 

 excepting the microscopic irregularity of the margins, either with 

 rare exceptions is entire (PI. X, a), or has merely a single, often 

 extended, lobe at base on one or each side (PI. XI, a), from which 

 it is sometimes partly separated by an incision (PI. X, c). 



The venation is pinnate. The blade's primary midvein is so 

 httle developed in the earliest form of the blade that the venation 

 at this stage of development may be called pseudo-flabellate, but 

 this midvein becomes evident while the blade is still simple. 

 A midvein is evident in all but occasional, very small, newly 

 formed, ultimate divisions of the later leaves. The two basal 

 primary branches of each midvein are usually much more com- 

 plex than the others. In the lobed leaf-sections one or each of 

 the two constitutes the venation of a lobe, according to whether 

 there is a lobe on one or on each side of the section. The develop- 

 ment of the branch into a midvein in the lobe is evident either 

 before or, if the lobe be very small, soon after the lobe has become 

 distinct from the leaf-section, and in any case before the lobe, as 

 a distinct pinna, becomes, if ever, lobed.. 



