THE PINNA-TRACE IN THE FERNS. 369 



here. The useful information which these Ferns give us is that their pinna-traces 

 departed from their leaf-traces at the points nearest to the pinnae. 



If we cany this into the living Ferns, any fundamental distinction between marginal 

 and extramarginal types of pinna-trace breaks down — the pinna-traces go off just 

 from the points which best serve the pinnse. The distinction between the marginal 

 and extramarginal types of pinna-trace must then be one due to the differences in 

 leaf-traces, not in the pinnae themselves. This is so far true. But a marked 

 divergence from the type general within a family, such as that of Balantium culcita 

 (pp. 350, 356) from the type of the Cyatheaceae, must make us pause in the generalisation. 

 We have in B. culcita a marginal type of pinna-supply, yet the general type in the 

 Cyatheacese is a very elaborate extramarginal one. The changes in the leaf-trace 

 of Gleichenia jiabellata throw some light on the interpretation of this divergence. 

 There the incurved margins of the leaf-trace round themselves off as circular 

 groups of tracheides which run up along the face of the gaps caused by the departure 

 of the pinna-traces (PL XXXIV. fig. 14), and then beyond the pinna-trace gaps unite 

 to the remaining tracheides of the median part of the leaf-trace (text-fig. 3). The 

 incurved edges of the leaf-trace in Gleichenia carry on the water-supply up the 

 adaxial face of the leaf. The pinnae are relatively large, and the pinna-traces make a 

 considerable drain on the tracheides of the leaf-trace. Apparently such a drain would 

 cause a serious deflection of water from the adaxial face of the petiole, and would 

 probably cause a water-starvation of the successive sets of pinnae above the basal 

 pair. The incurved hooks of the Gleichenia leaf-trace have thus the task of carrying 

 forward the water-supply from one pinna-gap to the next, and of providing sufficient 

 water to counteract the drain of several sets of pinnae. 



We would thus expect to find a considerable development of incurved hooks in 

 leaves possessing many pinnae, and perhaps most distinctly in those leaves with many 

 pinnae arranged in close succession (cf. Tansley, Ev. of Fil. Vase. System, p. 117, cf. 

 p. 126). In the Cyatheaceae the leaf generally possesses a long "tail" of lamina 

 beyond the last pair of actual pinnae. And in the Cyatheaceae the leaf-trace beyond 

 the last pair of pinnae is an ellipse of tracheides, thin on the adaxial face (cf. p. 360). 

 Just the same arrangement in the leaf-trace is found in Adiantum polyphyllum, 

 where the leaf ends in a long " tail " similar to that of Cyatheaceous leaf. Balantium 

 culcita has a much shorter leaf than the majority of the Cyatheaceae ; the pinnae are 

 relatively not heavy and are arranged at relatively considerable distances from one 

 another. Almost the same remarks apply to the leaf of Dichsonia fibrosa, which has 

 a marginal pinna-supply just like that of Balantium culcita. These are simple 

 Cyatheaceae (perhaps owing their simplicity to reduction) which have modified their 

 type of vascular supply to the pinnae in relation to their relatively short leaves. In 

 the leaf of the majority of the Cyatheaceae the complicated leaf-trace with its very 

 much incurved margins is related to the large size of the leaf, the weight of the 

 pinnae, and their large number. The incurved edges of the leaf-trace persist beyond 



