THE PINNA-TRACE IN THE FERNS. 



373 



opinion that the extramarginal type of supply is found on the whole among large- 

 leaved Ferns, the marginal type among the smaller-leaved genera. A few examples, 

 like Poly podium aureum (marginal) and Notholsena sinuata (extramarginal), can be 

 taken, which, if they stood alone, would prove the contrary proposition. The general 

 survey tends to reveal the extramarginal type among large-leaved, heavily pinnate 

 forms, the marginal type among the smaller-leaved, more lightly pinnate forms. 

 Davallia solida gives us an interesting example of a process which may help the 

 elucidation of the adoption of the marginal type of pinna-trace. The leaf-trace has 



h 



Text-Fig. 14. — Davallia solida (Forst. ) Sw. Series of diagrams illustrating the changes in the leaf-trace during the period 



of supplying the lowest pair of pinnae. 



two fairly large adaxial strands, with unincurved margins, and a small median strand 

 (text-fig. 14, a). The adaxial strand gives off a portion marginally on the side directed 

 towards the lowest pinna (text-fig. 14, b and c). This passes out into the base of the 

 pinna ; then another marginal portion is given off from the adaxial strand and follows 

 the first strand into the pinna (text-fig. 14, k). The pinna-trace is binary and gives 

 off two strands to the first pinnule. During the supply of the pinna by the leaf- 

 trace the median strand nips off a small strand while one of the adaxial strands is 

 preparing to give off its margin (text-fig. 14, b). This little strand passes to the 

 adaxial strand and fuses with it between the giving-off of the first pinna-trace-strand 

 and the second (text-fig. 14, h). The median strand itself moves towards the other 

 adaxial strand and fuses with it just before it nips off the first strand for the pinna 



on that side (text-fig. 14, e and f). Some of its tracheides remain as the abaxial 

 TRANS. ROY. SOC. EDIN., VOL. L. PART II. (NO. 11). 51 



