372 MR K. C. DAVIK ON 



llic same position in the rachis as do the parts of the leaf-trace which supply the 

 pinna of Matonia pectinata. In Archangiopteris, however, the details are a little 

 different, since the leaf-trace has a different outline from that of Matonia. One 

 strand which goes into the pinna comes from the side of the curve of the leaf-trace 

 set; the other comes from the abaxial face of the adaxial strand (text-fig. 13). The 

 terminal part of the adaxial strand remains to carry the water past this pinna-gap. 

 This terminal strand is itself reinforced below the departure-point of the pinnae by 

 internal accessory strands which come from the internal face of the abaxial curve. 



Wherever we find hooked adaxial portions in a leaf-trace, we do not find any 

 evidence of a process of reinforcement, prior to the departure of the pinna-traces, of 

 the parts of the leaf-trace supplying the pinnae. But the incurved hooks do vary 

 considerably in length and in their number of tracheides, according to whether 

 the leaf is a long one or a short one, one with heavy pinnae or one with light pinnae, 

 one with closely set pinnae or one with pinnae at considerable distances from one 

 another. 



PINNA TRACE , 



Text-Fig. 13. — Archangiopteris Henry i, Christ et Gies. Diagram illustrating the 

 method of departure of the pinna- trace. (After Gwynne-Vauuhan.) 



An excellent example of the type of hook found in a long leaf with heavy pinnae 

 is that of Didymochlsena truncatula (PI. XXXIII. fig. 7). Species of Dryopteris with 

 shorter leaves and lighter pinnae, like D. filix mas, have much smaller adaxial hooks. 

 In Microlepia hirta and M. platyphytta, where the pinnae are spread over approxi- 

 mately the same length of leaf, the adaxial hooks are of almost the same size ; in 

 M. speluncse, whose leaves are usually larger, the adaxial hook is larger than in either 

 of these. Leaves with relatively heavy pinnae, like Diplazium celtidifolium or D. 

 marginatum, have strongly developed adaxial hooks ; those with fairly light pinnae, 

 like the Blechnums, have weak adaxial hooks. In all of these the adaxial hook 

 appears to cany the water on from pinna-trace to pinna-trace along the leaf quite 

 satisfactorily. Whenever the adaxial hook disappears and we get a marginal supply 

 to the pinnae, the system which carried forward the water disappears. On the whole, 

 the appearance of the marginal type of pinna-supply seems to coincide with a reduc- 

 tion in the size or complexity of the leaf. This is by no means invariable, but a 

 contrast of such outstanding genera as Athyrium and Diplazium (extramarginal) 

 wit I) Asplenium and Ceterach (marginal), or of Ilypolepis and Adiantum (extra- 

 marginal) with Pellasa, Cheilanthes, and Cryptogramma (marginal), shows that there 

 i- 30me truth in the statement. A glance over the table on p. 354 confirms this 



