92 A. H. Graves, 
(2) Structure of Vascular Bundles. 
The axial bundle contains two or three annular tracheae in its 
young stage, which generally break down in the mature leaf to 
form an open passageway; surrounding this portion are phloem 
cells and parenchyma, difficult to distinguish on account of their 
low degree of differentiation, and about the whole axial bundle is 
a pretty definite endodermis, which Sauvageau has also noted. 
The structure of the two lateral bundles is extremely simple, 
consisting merely of a very few small conducting cells (Pl. VII, 
fig. 43, 76). 
d. Lacunae. 
On each side of the central bundle, separated from it by two or 
three layers of parenchyma, are the lacunae (Pl. VII, fig. 43, /a). 
In mature leaves these are of the shape of flattened cylinders, ex- 
tending longitudinally through the leaf, and divided into compart- 
ments by transverse, sometimes oblique, perforated, one-layered 
plates of roundish cells (Pl. VI, fig. 25; Text-figs. 14-15). The per- 
forations are round intercellular spaces, occurring regularly at the 


Fig. 15.—Surface view of one 
of the diaphragms which sep- 
Figure 14.—Longitudinal section of arate the leaf lacunae into com- 
lacuna, showing cross sections of partments. The small circular 
diaphragms; d@7, diaphragm; Za, la- outlines represent spaces be- 
cuna. >< 185, tween the cells. >< 350. 
angles of the cells and affording communication for the air or gases 
from one compartment to the next. 
e. Comparison with Other Potamogetonaceae. 
The anatomy of the leaf of Potamogeton pectinatus, a plant so 
much like Ruppia in external appearance, is almost a duplicate of the 
leaf structure of Ruppia (Schenck, 1886, p. 16 and figure 411 d). 
The other species of Potamogeton show differences more or less 
great, according to the shape of the leaf and the external conditions. 
The leaves of Zannichella palustris (Schenck, 1. c.) and Althenia 
