IX] 



THE PINNA-TRACE 



173 



by abstriction from the margin of the meristele. However unequal the division 

 may appear, it may still be held as an unequally developed dichotomy. 

 Such "marginal " origin of the pinna-trace is seen in Anemia and Loxsoina\ 

 and a good example is shown by Pteris umbrosa, R. Br. (Fig. 167). This may 

 be held to be the primitive method of supply of the lateral pinna, and it is 

 characteristic of leaves of moderate size. But in many leaves of large size 

 the meristele is strongly curved, and in these the origin of the pinna-trace 

 is not by abstriction from the margin, but from a point or points on the 

 abaxial or convex side of the curve. This has been 

 described as " extra-marginal," and a simple ex- 

 ample of it is seen in Dryopteris vivipara (Raddi), 

 C. Chr. (Fig. 168). Either of these types may be 

 further complicated in those Ferns where the 

 meristele is widened out and laterally grooved in 

 the manner already described for leaves of large 

 size (Fig. 161, 1,3). In them there may be vascular 

 connections of the pinna-trace with both abaxial 

 and adaxial curves of the meristele. This is illus- 

 trated for the marginal type in Lonchitis pubescens, 

 Willd. (Fig. 169), and for the extra-marginal type 

 in Loplwsoria pruinata (Fig. 170). 



There may be a good deal of variety of detail 

 in these attachments, and where the leaf-trace is 

 broken up into separate strands their recogni- 

 tion may be difficult. But they clearly follow lines 

 of convenience, the ends to be gained being first 

 a direct and effective supply for the pinna, and 

 secondly the maintenance structurally of a means 

 of conduction upwards to the higher region of the 

 leaf. The supply to each pinna is most readily 

 secured in small leaves where the leaf-trace is not 

 strongly curved by simple abstriction from the margin of the meristele. 

 But where it is strongly curved, as in larger leaves with numerous pinnae, 

 the attachment is most convenient on its convex abaxial face. A direct 

 supply upwards is at the same time secured by the continuance of the 

 marginal tract of the trace forwards, which thus forms a direct channel of 

 supply to the higher pinnae. 



The connections of the pinna-traces might hardly be expected to supply 

 reliable phyletic characters. In point of fact both the types above described 

 may be found in the same leaf in Trismeria trifoliata, where commonly the 

 supply to the larger, lower pinnae is extra-marginal, while that to the smaller 

 upper pinnae is marginal. This is exactly what m.ight be anticipated if the 



F"ig. 169. Diagrams illustrating 

 the marginal type of pinna- 

 supply with a "reinforce- 

 ment" derived from the ab- 

 axial curve of the leaf- trace, 

 as in Lonchitis ptibescens, 

 Willd. (After Davie.) 



