120 



BOTANICAL GAZETTE 



[august 



At the very tip of the stem the traces of the youngest leaves 

 ascend in an almost perpendicular direction about the region of 

 the so-called potential vascular tissue to the point where they 

 connect with the horizontal bundles. At this stage (fig. 10) all of 

 the girdling strands lie in substantially the same plane, the pair 

 associated with the youngest leaf describing slightly smaller arcs 

 than those of the older leaves of the same crown. As internal 

 radial growth and the appearance of new leaves crowd the older 

 parts farther and farther away from their original terminal position, 

 however, the lateral foliar traces become less vertical and more 

 oblique. With this radial and longitudinal expansion of the stem 

 is also associated a lengthening of the horizontal girdling strands, 

 and consequently a widening of the intervals and the arcs between 

 each lateral connecting trace. 



These leaf traces are always endarch and collateral as they leave 

 the stem cylinder, and also during their passage vertically and 

 horizontally through the cortex to a point well up in the leaf base. 

 They are so orientated that the xylem and phloem are directed 

 toward the inside and the outside of the stem respectively. Trans- 

 verse and longitudinal views (fig. 1 2) throw additional light on the 

 organization of these cortical bundles. 



Course of leaf trace in adult stem. — Due to the diffi- 

 culties involved in following up strands of such size, it is impossible 

 to determine with certainty whether the arrangement of the leaf 

 traces in the adult stem of D. spinulosum is the same as that found 

 characteristic of the seedlings. The problem becomes increasingly 

 difficult as the plant reaches an age when the crown comprises 

 numerous developing leaves. From longitudinal and transverse 

 sections of the adult stem, however, it is evident that the same 

 general relation between lateral oblique traces and a horizontal 

 girdling strand is maintained, but it is probable that the girdling 

 is only partial, that traces a and a 1 (fig. 9) would have their origin 

 at points more remote from each other in the adult stem. It is 

 also probable that there is no appreciable increase in the number 

 of traces associated with successive leaves, beyond the number 

 described as supplying the leaves of the ten-year-old seedling 



(fig- 10). 



