7i6 A TEXTBOOK OF THEORETICAL BOTANY 



The trace first passes obliquely outwards through the cortex, then it 

 divides and the two halves diverge, each running round in opposite directions 

 in the outer cortex, gently rising and meeting at the base of the leaf, where 

 they bend sharply out into the petiole. These are the girdle bundles. In 

 their course they cross the girdle bundles of each of the other leaves in their 

 own group, and there is a distinct gap longitudinally between the girdle 

 systems of successive leaf groups, which are, however, linked by anastomosing 

 bundles. Leaf traces, called radial traces, also arise from other points 

 around the vascular ring. These also bifurcate, and each half produces a 

 complex system of anastomosing branches, which are attached to the girdle 



Fig. 719. — Liriodeiidron tulipifera. Model 

 of the vascular structure at a stem 

 node showing girdle development of 

 the leaf traces. The upper leaf trace 

 bundle gives off short stipular traces. 



bundles both of their own and of other leaves in the same group. The leaf 

 traces of each group of leaves thus altogether form a single united system. 



The two girdle bundles, each augmented by additions from the radial 

 bundles of the system, enter the leaf base and there they branch freely, 

 the separated branches arranging themselves in the horseshoe pattern 

 characteristic of the petiole. 



A somewhat comparable system of leaf traces occurs in Liriodendron and 

 some other members of the Magnoliaceae, a fact which may possibly indicate 

 a Cycadean relationship for this group of Angiosperms (Fig. 719). As in 

 Cycads the girdling is absent from the flower structures. 



The xylem of the girdle bundles faces inwards. They are usually of 

 collateral structure, but in Cycas concentric bundles occur, each with its own 



