444 Holm. — Bartonia , Muehl. An Anatomical Study . 
portion of the stem is occupied by a thin-walled but compact pith with no 
deposits of starch. 
If we examine one of the internodes from the middle of the stem, the 
structure is somewhat different. The cross-section is quadrangular, and 
stomata are more frequent (Plate XXXIV, Fig. 23) ; they are surrounded by 
four cells. A typical, thin-walled endodermis is differentiated in this part of 
the stem, but otherwise the structure of the cortex and epidermis is identical 
with that of the basal internode. The mestome-bundles are very irregular 
(Plate XXXIV, Fig. 20), some being bicollateral, others being reduced to 
a small group of vessels, bordering directly on the endodermis (Fig. 24), 
and the hadrome does not here form a closed ring around the pith. We 
noticed six groups of leptome in the pith, and these were more or less 
separated from the hadrome, as may be seen in our figure (24). 
In the apical internodes we notice the same structure and the same 
irregular development of the mestome-bundles. For instance, as shown in 
Fig. 12, pure leptomatic strands occur within the endodermis, or the 
mestome-bundles may be strictly collateral, though exceedingly small, as 
figured on Plate XXXIII, Fig. 13. 
It thus seems characteristic of this species that the stem lacks 
mechanical tissues, stereome and collenchyma, and that epidermis is the 
only tissue that has acquired some strength and firmness by the thickening 
of the cell-walls. 
The Leaf. 
All the leaves of Bartonia verna are minute, subulate and appressed 
to the stem ; they are triangular in cross-sections with a keel on the dorsal 
face. The cuticle is thick, wrinkled on both faces of the blade. Viewed 
en face , the cells of epidermis are rectangular with the radial walls slightly 
undulate ; in transverse sections the cells on the dorsal face are con- 
spicuously larger than on the ventral ; the outer cell-wall is moderately 
thickened on both faces. Stomata surrounded by four cells were observed 
on both faces, but they are most frequent on the dorsal. The chloren- 
chyma represents a homogeneous tissue of roundish cells loosely connected, 
and often traversed by a lacune near the centre ; chlorophyll, but only in 
small quantity, was observed. There is only one vein, located in the keel ; 
it consists of a small mestome-strand, destitute of mechanical support, and 
has no parenchyma-sheath either. The elements of the leptome are very 
small, and the hadrome contains only a few, narrow vessels. 
Glandular hairs of the same structure as those which we observed in 
Obolaria ( 1 . c.) were noticed at the very base of the leaves, but only on the 
ventral face. 
