The Morphology of Ruppia Maritima. 119 
longitudinal rows present in the leaves, receive the special name 
“Jacunae.” 
The function of these lacunae has not yet been sufficiently in- 
vestigated. So far as is known, they occur in all water plants. 
Schenck (1886, p. 49) states that water plants grown on land diminish 
the size of their air spaces and, conversely, land plants grown under 
water reveal a tendency toward a loosening up of the cortical 
parenchyma. This necessarily indicates that the formation of air 
spaces is in some way connected with and necessitated by an 
aquatic environment. Haberlandt (1896) has probably hit the truth 
of the matter when he explains that this is a method of obviating 
the difficulty of osmotic interchange of gases in submerged plants: 
“Es wird eine “innere Atmosphare ” geschaffen, mit welcher die ge- 
schiedenen Gewebe einen lebhaften Assimilations- und Atmungs- 
gaswechsel unterhalten.” The larger these inner air reservoirs are, 
so much less will the plants have to suffer the difficulties of direct 
interchange of gases with the surrounding water. It is probable 
also that the considerable amount of air and gases enclosed in these 
intercellular spaces is of advantage for submerged as well as for 
floating plants, by its lessening of the specific gravity of the plant. 
c. Absence of Mechanical Tissue. 
As has been, in part, pointed out above (see p. 116), a land plant 
with dimensions of stem, branches and leaves similar to those 
occurring in Ruppia, must of necessity develop considerable mechani- 
cal tissue. But in no part of the vegetative organs of Ruppia is a 
characteristic thick-walled tissue developed. That such mechanical 
tissue is entirely absent is to be explained in the greater supporting 
capacity of water as compared with air, resulting from the greater 
density of the aqueous medium. In this way this lack of supporting 
tissue is to be considered as an adaptation to environment. 
d. Reduction of the Vascular System. 
The vascular system is greatly reduced. If we except the minute 
cortical bundles of the stem and the small lateral leaf nerves, the 
entire vascular system of the shoot is represented by a single axial 
vascular strand. The xylem portion mostly disappears in the mature 
stages except at the nodal regions, leaving a central cavity in the 
vascular area. The phloem portion is, however, in all cases intact, 
consisting of sieve tube#*companion cells and phloem parenchyma. 
One may lay this vascular reduction entirely to the different mode 
of nutrition employed by submerged plants. On the one hand, the 
food solutions formerly conveyed from the roots by way of the stem 
are probably now absorbed in large part from the surrounding water, 
