80 A. H. Graves, 
Sauvageau, 1891 1; 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 (PI. II], 
fig. 8). No sieve tubes can be distinguished (Text-fig. 5). Ata 
later stage (Pl. 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. >< 210. cular elements. >< 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 xylem 
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, wherethey 

7 we» are conspicuous in 
APRA © ‘a the mature stage. The 
- resulting space, then, 
Figure 7.—Tracheae and tracheids from axial vas- isa lysigenous cavity, 
cular bundle of stem: a. tracheid with annular 
thickenings ; b. tracheid with annular, spiral, and does notrepresent 
and pitted thickenings; c. disorganized trachea an enlarged cell (PI.I, 
with thickenings lying in the cavity. >< 1350. fig.3). The thicken- 
ings of the tracheae 
are mostly annular, although spiral and other forms may occur 
(Text-fig. 7). Occasionally these thickenings may be found lying loose 
