The Morphology of Ruppia Maritima. 127 



2. The hydrophytic adaptations of the shoot are as follows : first, 

 in its external form it shows a weak, wide spreading branch system, 

 with slender stems and long grass-like leaves ; second, in its internal 

 structure, it reveals a thin-walled epidermis, photosynthetic and 

 without stomata, in the leaves, a production of slime by the axillary 

 scales for protective purposes, a copious internal development of 

 air spaces, a complete lack of mechanical tissue and a reduction of 

 the vascular system to a single main axial bundle and two small 

 lateral bundles in both stem and leaf. 



3. The hydrophytic adaptations of the root are as follows : first, 

 in its external form, a reduction of the root system to small, un- 

 branched, adventitious roots, borne singly at the nodes, and the 

 formation of a protective coleorrhiza ; second, in its internal structure, 

 the presence of numerous air spaces and the reduction of the 

 vascular system to a single, axial, concentrically arranged bundle 

 similar to that in the shoot. 



4. The characteristic adaptations of land halophytes are wanting 

 here, for the reason that their cause, the need for reduction of 

 transpiration, is lacking. 



5. Ruppia shows, however, a remarkably interesting lialophytic 

 adaptation in its power to live in salt water, which, when applied 

 to submerged fresh water plants, causes instant plasmolysis. 



6. This salt water is not equal in strength to that of the open 

 ocean, containing in New Haven Harbor, about 2.8 per cent of salt, 

 and may therefore be termed " brackish ". 



7. My experiments show also that this power is confined to 

 a very slight margin, i. e., that slightly concentrated harbor water 

 causes plasmolysis in both root hairs and leaf cells of Ruppia prov- 

 ing that the plant, as now constituted, could not exist in ocean 

 water. 



8. The tracheae and cortical bundles are evidently more or less 

 rudimentary and possibly represent useless structures handed down 

 by heredity. 



