174 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



sympodial construction of the leaf be really true, and its progressive modifi- 

 cation according to convenience be as above stated. As the transition occurs 

 towards the apex to the primitive dichotomy, there 

 should also be a transition to the primitive marginal 

 origin of the leaf-trace; and this is what may actually 

 be observed in the leaf of Trismeria. 



Comparison of the pinna-traces in various groups 

 of Ferns has shown, nevertheless, that there is greater 

 uniformity than might have been expected. The more 

 primitive marginal type is found constantly in the 

 Schizaeaceae, Pteridinae, and Polypodiinae, and in 

 many Davallieae, Asplenieae, Gymnogramminae, and 

 Cheilanthinae. The extra-marginal is probably deri- 

 vative, and is characteristic of the larger leaves of the 

 Osmundaceae, Gleicheniaceae, Hymenophyllaceae, 

 Blechninae, and Onocleinae ; while it occurs also in 

 many of the Dicksonieae, Cyatheae, Woodsieae, and 

 Aspidieae. Taking these facts together, the extra- 

 marginal type appears to be essentially a consequence 

 of greater size and elaboration of the leaf, with which 

 the curvature of the meristele is so closely related. 

 In certain alliances, however, the type of origin of 

 the pinna-trace appears to be so far fixed as to give 

 it a certain value for phyletic comparison. 



Venation 



The course of the veins in the flattened blade has 

 already been discussed, and its value for purposes of 

 comparison considered. Little need be said on the 

 anatomical detail of the vascular system as it ap- 

 proaches the distal end or margin of the leaf, and 

 resolves itself into the ultimate veins. These remain 

 surrounded by endodermis, which encloses a larger or 

 smaller mass of xylem directed towards the adaxial 

 surface, and of phloem directed towards the abaxial. 

 Technically the ultimate strands are collateral, and 

 mechanical tissue is often associated with the strand, 

 in girder form. \\\ fact the characters of the ultimate 

 veins in Ferns resemble in essential points of struc- 

 ture those in the lamina of other leaves, or that of 

 the trace of the primordial cotyledon. 



It thus appears that the vascular system of the 



Fig. 170. Series of trans- 

 verse sections through the 

 rachis of Lopkosoria, suc- 

 cessively from below up- 

 wards, showing the origin 

 of a pinna-trace. ( x 4. ) 



leaf offers characters 



