The Morphology of Ruppia Maritima. 119 



longitudinal rows present in the leaves, receive the special name 

 "lacunae." 



The function of these lacunae has not yet been sufBcientl}' in- 

 vestigated. So far as is known, they occur in all water plants. 

 Schenck (1886, p. 49) states that water plants grov^m on land diminish 

 the size of their air spaces and, conversely, land plants grown iinder 

 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 Tissvie. 



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 tubes, 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, 



