IX] 



THE PHYLLOPODIUM 



165 



tract of xylem. This is seen in Angiopteris{l.c. Fig. 130), Danaea(^\%. 131), 

 and in Kaulfussia (Fig. 137, D). Still there is an underlying truth in Sinnott's 

 generalisation, which has its interesting bearing on ontogenetic comparison. 



The distance through which the protoxylem may be traced downwards 

 into the axis varies according to the proportions. Where the axis is short 

 with crowded leaves it may be impossible to follow the protoxylem onwards 

 into the stele. This is a natural consequence of the fact that " protoxylem " 

 is the structural expression of development during extension of tissues. If 

 little or no extension occurs, an absence of xylem structurally defined as 

 "protoxylem" may be expected. 



A comprehensive view of the leaf-traces of Ferns leads then to the 

 generalisation that the primitive leaf-trace was structurally very like the 

 protostele of the axis, with a single protoxylem-group. That losing its cylin- 

 drical form it became flattened, with various modifications of outline. That 

 in the majority of Ferns, and especially in the Leptosporangiate Ferns, it 

 adopted the form of an adaxially concave strap, curved like a horse-shoe as 

 seen in transverse section, and with a pluralit)' of protoxylem-groups on the 

 adaxial face of the xylem. But that in the most advanced types of them 

 this strap became disintegrated as a " divided 

 leaf-trace," though its parts still show by their 

 arrangement the underlying horse-shoe curve. 

 Thus the characters of the leaf-trace provide 

 important features for phyletic comparison. 



The PHYLLOPODIUM 

 In Chapter V the origin of the phyllopodium 

 has been traced. In the great majority of Fern- 

 leaves it extends from the leaf-base to the distal 

 tip, appearing below as the petiole, and above as 

 the rachis or midrib. It is traversed throughout 

 by the vascular supply. As the leaf-trace passes 

 outward to the petiole the meristele very frequently 

 dilates, especially in relatively primitive forms 

 (compare Fig. 155). Attaining its greatest size 

 and complexity near to the base of the petiole, it 

 is gradually attenuated upwards to the leaf-tip. 

 Where there is an undivided trace there is often 

 little to record. This is the case in Loxsoma. Here 

 the curve of the trace is of rather open form, the 

 xylem showing the usual adaxial hooks. There 

 are six protoxylem-groups, of which two occupy 



Fig. 158. Acropetal series of sec- 

 tions of the meristele of Z^x.>-(?wa, 

 showing its modifications to- 

 wards the apex of the leaf. 

 (After Gw-ynne-Vaughan. ) 



the adaxial angles of the curve (Fig. 158, a). Higher up the curve flattens {b). 



