STRUCTURE. 



phloem below. Under this is a double bundle with xylem in the middle 

 and phloem above and below, then another double bundle, placed trans- 

 versely, and finally a simple bundle with xylem uppermost. The lowest 

 bundle is underlaid by a transverse band of thin yellowish collenchyma. 

 Near the apex of the same midrib the lowest bundle with its band of 

 collenchyma still persists ; above it is an inverted simple bundle, and 

 above this again, near the base of the palisade layer, is a simple bundle 

 with normal orientation. Lateral bundles seem to arise along the midrib 

 from the median double bundles, and as they go farther from the rib they 

 swing round so as to present a normal orientation of xylem and phloem. 

 At a point nearer the petiole than the section first described, the place of 

 the transverse double bundle is taken by an irregular vascular mass, 



o 



d 



FIG. 31. Arrangement of vascular bundles in veins of leaf: (a) section near the base; (l) near apex 

 of midrib, IV. odorata ; (c), (d) from N. ciijtenste X ; e, upper epidermis ; p, parenchyma ; pa?, palisade 

 layer ; pe, lower epidermis and parenchyma ; ph, phloem ; sp, spongy parenchyma ; z, xylem. 



consisting of one or two xylem areas, each surrounded by two to four 

 phloem lobes ; each phloem lobe represents a bundle which emanates from 

 the vascular collar, several of which fuse to form the transverse double 

 bundle of the rib. 



The primary veins are traversed by a lower large simple bundle with 

 phloem below and xylem above, and a similar smaller bundle above this. 

 In all of the lesser veins the orientation of bundle tissues is strictly normal. 

 The lower curve of the vein also is lined within with corner-collenchyma, 

 but there is no interruption in the palisade layer above ; the bundle has a 

 close sheath of small cells, and a wider investment of parenchyma. In all 

 but the finest ramifications the veins lie nearer to the lower surface of the 

 leaf and are made fast there by the continuity of their surrounding 

 parenchyma with that which lines the lower epidermis ; for there is such a 

 lining layer near the veins, even in N. rubra. The smallest veins lie just 

 beneath the palisade layer (Fig. 30, v), where the exchange of crude and 

 elaborated materials takes place. I have not observed any free endings 

 of veins ; anastomosis seems to be universal. 



