606 BRITTON: VEGETATION OF THE 
Diameter of root, 25 mm. 
«¢ — « central cylinder (oval), 2x.3 mim: 
«¢ _-* Jargest vessels, 20 fl. 
Merzomia sp. —The primary root shows three large vessels 
and three smaller ones in the bundle which seem to be entirely 
surrounded by phloem. The phloem contains several strands of 
sclerenchyma-cells with closed lumina. The endodermis is com- 
posed of oblong thin-walled cells. The pericycle consists of simi- 
lar but smaller cells. The cortex has very large cells with thin 
undulating walls and some intercellular spaces. Considerable lig- 
nified tissue is formed in the secondarily thickened roots and the 
vessels are large. 
Diameter of root, .31 mm. 
‘« central cylinder, .18 mm. 
s¢ _§* largest vessels, 30 LW. 
Cassia CHAMAECRISTA.— Most of the preserved material 
shows secondary thickening in the roots, but the root from a 
young seedling exhibits the primary structure. In the latter there 
are four large vessels, one in the center and the others around it 
forming a three-pointed xylem in transverse section. This is sur- 
rounded by the phloem. Endodermis is prominent, of good-sized 
cells, oblong to circular in cross-section, and with walls only 
slightly thickened. Just inside the endodermis and near the three 
radiating xylem points are three strands of sclerenchyma-cells. 
The lumina are nearly closed and the lamellae can be distinguished. 
The cortex is composed of large parenchyma-cells with thin undu- 
lating walls. 
Diameter of entire seedling root, -3 mm. 
“© ‘central cylinder, .16 mm. 
«« «largest vessels, 66 pu. 
In roots which have undergone secondary thickening, the veS- 
sels are large and numerous, varying from thirty-six to fifty in the 
sections examined and probably even more in larger roots. These 
vessels are pitted, approaching scalariform. About half of the 
small xylem-cells in the center between the vessels stain dark after 
standing for a long time in chloriodide of zinc, though these were 
not stained at first by this reagent. They are probably parenchy™4 
xylem-cells with cellulose walls which stain only after standing 
