1907] HOLM—RUBIACEAE 173 
quantity. The innermost layer of the cortical parenchyma is differ- 
entiated as an endodermis (fig. 21, end), in which the inner and 
radial walls are very heavily thickened. The central cylinder con- 
tains a closed ring of leptome and about four rows of very narrow 
scalariform vessels and medullary rays of but one row between each 
two mestome strands. A thin-walled, solid pith occupies the center 
of the stem. Cells with raphides are frequent in the stem, but it 
seems as if they are confined to the cortex. 
The leaves.—As mentioned above, the leaves are held in a hori- 
zontal position, but nevertheless their structure is almost isolateral, 
so far as the distribution of stomata and the differentiation of chloren- 
chyma are concerned. The cuticle is quite thick and smooth on 
both faces of the blade except the midrib, where it shows prominent 
longitudinal striations. Viewed en face the epidermis consists of 
pentagonal or hexagonal cells on the upper face, but of more irregular 
cells on the lower; the lateral cell walls are straight and not undu- 
late in any part of the leaf. The stomata are most frequent on the 
dorsal face (fig. 19); the subsidiary cells, normally one pair, some- 
times occur one on one side of the stoma and two on the other side, 
but all parallel with the stoma (fig. 20). Similar irregularities were 
also noticed in some of the other Rubiaceae examined, as may be 
seen from my figures. Epidermal projections abound in Diodia; 
the most common are small warts (figs. 17 and 18) developed merely 
as protuberances of the outer cell wall; they are very minute, but so 
numerous that they render the leaves scabrous. Unicellular, rather 
thin-walled, pointed hairs are also frequent (jig. 19) on both faces of 
the blade; and along the margins are curved, thick-walled, spine- 
like projections of exactly the same structure as those well known 
on the leaves of Gramineae and Cyperaceae (fig. 18). 
A cross-section of the leaf shows a large-celled epidermis on both 
faces, with the outer wall slightly thickened on the upper; the warts 
appear most numerous on the upper face. The position of the sto- 
mata differs on the two surfaces, being raised on the lower and level 
with the epidermis on the upper. The midvein shows a prominent 
group of collenchyma and thin-walled water-storage tissue on the 
leptome side, while no collenchyma was observed above the hadrome. 
An isolated strand of collenchyma occurs also in the margins of the 
