BY il. B. WELCH. 



477 



In some cases one or both of these zones may be divided into smaller areas 

 {see PI. xliii., %. 2), but as a rule the equal development of the sylem on both 

 sides is a characteristic of these species. Text-flg. 1 g shows the upper zone of 

 xylem divided intt> two, this section being taken at a point 5 inches from the 

 junction of the petiok and laiuina. In this section the ducts are 0.04 nim. in 

 diameter and disappear a very short distance further along the leaf. In the 

 uppermost three inches of the leaf, represented in Text-fig. 1 by the 3 sections 

 h, i, j, the ducts are absent, and the two smaller xylem areas also disapjiear in 

 the last section. The only marked exception to this typical arrangement of the 

 xylem is found in the mid-rib of Angophora lanceolata (Fl. xliv., fig. 6). The 

 structure in this species is, however, typical of the vast majority of the 

 Eucalypts examined, Ijut in wiiich ducts are wanting. 



There is a gradual diminution in diameter a.s the canal passes along the 

 petiole and through the lamina of the leaf. Where the petiole joins the stem 

 the ducts are comparatively large, varying up to 0.35 mm. in diameter in E. 

 eximia. At the junction of tlie petiole and leaf blade they are somewhat smaller, 

 and in tiie leaf are comnumly 0.10 mm. in diamet<'r, showing a gradual reduction 

 in size until at a point about 2/3 of the distance I'rom l)ase to apex, they 

 disappear. 



Although two is the niM'mal number of ducts in tlie leaf, it is quite common 

 to find a third or fourth, seldom more. These arise independently alongside the 

 others, in some cases in actual contact (Text-fig. 2). but evidence of branching 

 is rarely found. These secondaiy ducts do not persist, as a rule, for any 

 distance, and gi-adually taper off and disappear. In a few cases one of the 



^^) 



Text-fig. 3. — Transverse sections of the junction of the stem and petiole of a 

 mature leaf showing diagrammatic arrangement of the xylem and ducts. 



main ducts may disappear, but only for a short distance, reappearing in the 

 same relative position a-s before. The fact that the two leaf ducts are nor- 

 mally continu«us for some distance is shown by the quantity of the exudation 

 from them when cut transversely. This is particularly noticeable in the lai-ge 



