i66 THE VASCULAR SYSTEM OF THE LEAF [CH. 



and the strands of tlie pinnae are nipped off from its margins. Higher again 

 the hooks disappear, the protoxylems becoming fewer until only three are 

 left, one at the end of each arm, and one in a median position (c). Further 

 up the curve again contracts, the meristele taking a triangular outline (d), 

 and finally an oval form, with completely collateral structure and a single 

 protoxylem (e). This may be taken as the history of a foliar strand in any 

 relatively primitive example. An essentially similar structure is seen in 

 Schizaea, Anemia, Plagiogyria, some Davallias, Onychium, Cheilanthes, and 

 Pellea, all being Ferns of moderate size. A like form is also found in the 

 pinna-supply of larger Ferns. It may be taken as a very general type for 



I4k 

 Fig. 159. Transverse section of the meristele from the petiole of Davallia speluncae. 

 6' = endodermis; / = pericycle; //4 = phloem; ///i = protophloem ; /rx = protoxylem ; 

 f/ = cavity parenchyma; /(^/{■ = adaxial hooks. (After Gvvynne-Vaughan.) 



the more primitive Leptosporangiates where the leaves are small, or for the 

 pinnae or pinnules of larger Ferns. The detail of such a meristele is shown 

 in Fig. 159, for Davallia speluncae, which corresponds very nearly to that 

 seen in stage {b) of the above figure for Loxsoma. Athyrium and Asplenijmi 

 show another type of change by separation of the trace in a median plane 

 into two halves, at the base of the petiole (Fig. 157). A like condition 

 is seen in Gy mnogr amine japonica (Fig. 160), and in Matteiiccia, Struthio- 

 pteris, Onoclea, Dryopteris phegopteris, Scolopendrium, and many others (see 

 Studies, IV). This has been styled the Onoclea-type by Bertrand and Cornaille. 

 Further up the phyllopodium the two halves unite, sometimes reconstructing 



