608 BRITTON: VEGETATION OF THE 
cal cells are rather large, with thin undulating walls and very 
small intercellular spaces. The outer layer has cells of much 
more regular shape than the others, and some are prolonged into 
unicellular root-hairs. Throughout the whole root but especially 
numerous in the center are certain cells filled with a dark-brown 
substance, resembling tannin, but which is probably a resin, as 
Bary refers to resinous substances in the roots of other members 
of the genus.* Roots which have undergone secondary thicken- 
ing show a large group of thick-walled, probably lignified cells in 
the center, outside of which is a zone containing the xylem and 
phloem patches. Running through this zone are radial strands 
of lignified cells and one or two rows next to the cortex. The 
cortex is made up of cells which are oval or oblong in cross- 
section with intercellular spaces. With the exception of the outer 
layer which has colorless cells, the cell-walls are brown in color 
though none of the cells as in the primary root are filled with 
tannin. 
Diameter of root, .65 mm, 
** central cylinder, .25 mm. 
es “* largest vessels, 23 pe. 
Rhizome : The central portion is made up of pith-cells nearly 
circular in cross-section. Some of these are colorless but many 
are filled with small brown granules, while others are entirely 
filled with the brown substance. A thin layer of small-celled 
sclerenchymatous tissue surrounds the pith. The cortex is formed 
of large circular cells, much resembling the pith-cells, and many 
contain resin. Large intercellular spaces occur in the cortex. 
Half way from the sclerenchymatous layer to the periphery is a 
row of ducts or intercellular passages containing resin. The out- 
side of the rhizome is covered with what appear to be simple 
hairs. 
ASCLEPIAS VERTICILLATA, — The bundle of the root contains 
no large vessels, but there are many small ones. Endoderm!s 
can be made out in specimens studied, though it is evident that 
secondary thickening of the root has commenced. The cells are 
oblong in transverse section and have thin walls. The xylem has 
——— 
jer Phanero- 
* Bary, A. de. Vergleichende Anatomie der Vegetationsorgane der Phane 
gamen und Farne, 219. 1877. 
