THE PINNA-TRACE IN THE FERNS. 



351 



The " extramarginal " types differ from them in leaving the leaf-trace margins 

 intact throughout the petiole. In Didymochlsena truncatula (Sw.) J. Sm. (PI. XXXIII. 

 fig. 7), the leaf-trace is composed of several strands. The two adaxial strands have 

 their margins sharply recurved, so that they are almost parallel to the adaxial 

 surface of the petiole. To supply the basal pinna, the back of the hook is lengthened 

 towards the adaxial corner of the petiole. Across the narrow space between the 

 two strips of tracheidal tissue thus formed new tracheides spread. The original 

 outline of the leaf-trace strand is thus re-formed, while a ring of tracheides is attached 

 to the back of its hooked portion. This ring soon separates, to pass into the base of 

 the pinna and divide up into a pinna-trace resembling the leaf-trace. The margin of 



Text- Fig. 1. —Balantium culcita (L'Herit. ) Klf. Successive stages in the development and liberation of the pinna-trace. 



the parent trace remains intact during the process. Since the pinna-trace goes 

 off from the outside of the parent strand, the method of supply may be termed 

 " extramarginal." 



The process of supplying the basal pinnae is exactly the same in leaves with 

 unbroken leaf-traces, such as that of Odontosoria chinensis (L.) J. Sm., var. Veitchii 

 (PI. XXXIV. fig. 9). 



In this species and in Didymochlxna truncatula no gaps are left in the tissues 

 of the parent trace when the pinna-trace is given off. Sometimes, however, the 

 departure of the pinna-trace makes a gap in the tissues at the back of the hook in 

 the leaf-trace. This is readily seen in Leptopteris hymenophylloides (A. Rich.) Pr. 

 (PI. XXXIII. fig. 8). Hymenophyllum demissum (Forst.) Sw. (PI. XXXIV. fig. 10) 

 gives off the supply to the basal pinnae in the extramarginal manner. The tracheides 

 of the leaf-trace do not form a perfectly continuous series, but the marginal set, 

 which terminate the hook of the leaf-trace, remain in position when the pinna-trace 

 departs. It can hardly be said that a gap occurs opposite to the departing pinna- 

 trace, for a single tracheide occurs between the end of the abaxial curve and the 



