80 



A. H. Graves, 



Sauvageau, 1891 I ; Chrysler, 1907) are similar to those of Ruppia, 

 but are continuous up and down the cortex and send a branch to 

 the central cylinder at the nodes. 



(b) Structure of Vascular Bundles. 



Anatomically the cortical bundles in a young stage consist of small, 

 closely packed cells, which are all tracheids or tracheae (PL III, 

 fig. 8). No sieve tubes can be distinguished (Text-fig. 5). At a 

 later stage (PI. I, fig. 5 ; Text-fig. 6) these small elements have for the 



Figure 5. — Cross section of cortical Figure 6. — Cross section of cortical 



bundle., before dissolution of its bundle, after dissolution of its vas- 



vascular elements, x 210. cular elements, x 300. 



most part been resorbed, so that only a passageway remains, except 



at the nodes, where they remain intact. 



In structure, the central cylinder is composed of a central X3dem 



region surrounded by a zone of phloem. The xylem is in a young 



stage represented by 

 a group of tracheae 

 at the center, which, 

 however, very early 

 in the development 

 of the stem, become 

 pulled apart and dis- 

 organized except at 

 the nodes, where they 

 are conspicuous in 

 the mature stage. The 

 resulting space, then, 

 is a lysigenous cavit}-, 

 and does not represent 

 an enlarged cell (PI. L 

 fig. 3). The thicken- 

 ings of the tracheae 



are mostly annular, although spiral and other forms ma}- occur 



(Text-fig. 7). Occasionally these thickenings may be found lying loose 



Figui'e 7. — Tracheae and tracheids from axial vas- 

 cular bundle of stem: a. tracheid -with annular 

 thickenings ; b. tracheid with annular, spiral, 

 and pitted thickenings ; c. disorganized trachea 

 with thickenings lying in the cavity, x 1350. 



