ZYGOPTERIS 311 



this region the central tissue of the trace contains small 

 tracheides, continuous with the internal tracheal group 

 of the stele. At the two external angles are the groups 

 of small elements which no doubt constitute the 

 protoxylem of the foliar bundle. 



As we follow the leaf-trace outwards through the 

 cortex, we find that it divides into two strands, of very 

 different form, both lying on the same radius (see Fig. 

 115, l.t., where this division is just beginning). The 

 outer of the two strands is the foliar bundle, which 

 is continuous with the external side of the original 

 triangular strand, while the inner strand is destined for 

 the axillary shoot. We will first follow the course of 

 the foliar bundle, which, as it separates from the inner 

 strand and passes out into the petiole, gradually assumes 

 a somewhat complex form. At first the bundle 

 consists simply of a tangential, somewhat bent, plate of 

 wood, surrounded by phloem, and spreading out a little 

 at either extremity, but in its outward progress these 

 lateral expansions increase in importance, so that the 

 ultimate form, in transverse section, is that of the letter 

 " H," the cross-stroke of the H being placed tangentially 

 with reference to the parent stem (cf. Fig. 118, from 

 another species). The largest tracheides are in this 

 middle band ; the smallest, constituting the spiral 

 protoxylem, are at the outer edge of the side-pieces. 

 The phloem surrounds the xylem and follows its 

 contour. This H-shaped petiolar bundle is common, 

 with slight modifications, to all species of Zygopteris, 

 and, in fact, constitutes the character on which the genus 

 was founded. 



The other bundle, which is given off on the inner 



