90 



LEAF-ARCHITECTURE OF FERNS 



[CH. 



distal branching and venation of Fern-leaves which do not show helicoid 

 branching throughout. For instance, the \edi{oi Pteris semipinnata as a whole 

 is pinnate, but the pinnae themselves show scorpioid branching (Fig. 82, E). 

 Where the scorpioid structure is strongly developed with a marked 

 phyllopodium, a succession of alternating pinnae is commonly produced, 

 as in the Male Shield Fern, or any other of 

 our ordinary native types. In such cases, if 

 the actual development of the individual 

 leaf be traced, the primordia of the earlier 

 pinnae are found to arise below the grow- 

 ing apex of the leaf, appearing as lateral 

 outgrowths upon it (Fig. 83). The branch- 

 ing is thus in point of fact monopodial. 

 But as the apex of the leaf is approached 

 there is a gradual transition to sympodial 

 dichotomy, and finally it may be to equal 

 dichotomy of the apex (Figs. 84, 85). The 

 same may also be found in the relation of 

 the pinnae and pinnules. In the ontogeny 

 of the leaf there is in fact in the early 

 stages a monopodial branching, which is 

 characteristically a later type of branching : but in the later stages a tran- 

 sition may be seen to that branching which is characteristic of the juvenile 



Fig. 83. Young leaf of Ceratopteris seen in 

 surface view, after Kny; showing apical 

 segmentation. It is clear that the alter- 

 nating pinnae, which arise monopodially, 

 do not correspond to the segments cut off 

 from the apical cell. 



Fig. 84. Portion of the leaf-surface of a sporeling of Aspleniuin serpenlini, 

 showing how dichotomy accompanies the marginal growth, (x 190.) To 

 the left a diagrammatic representation of the same. (After Sadebeck.) 



leaves. Consequently, if the phyletic history be reflected in these changes 

 in the method of branching, the series would have to be taken in reverse of 

 that actually seen in the development of the individual leaf The evolu- 

 tionary succession of steps would then read thus : (i) equal dichotomy ; 



