122 A. H. Graves, 



On account of this difficult}- in water absorption, then, one finds 

 exhibited in land halophytes man}' water-storing- devices and typical 

 xerophytic methods of diminishing transpiration. 



The water halophytes, on the contrary, especially the submerged 

 aquatics, such as Ruppia, Zostera, Phyllospadix, &c. — members of 

 the Enalid hydrophytic society of Warming (1902, p. 156) as above 

 noted— evince none of these modifications. The hydrophytic adap- 

 tations already described for Ruppia show how closely this plant 

 resembles a typical submerged fresh water hydrophyte. Although 

 surrounded by salt water to a much greater extent than land 

 haloph}'tes, yet it shows none of the characteristic xerophytic mod- 

 ifications which are associated with land halophytes in general. 



The explanation of this may be based on the fact that trans- 

 piration, as such, does not appear in Ruppia. The comparatively 

 small amount of water absorbed by the roots, the absence of any 

 openings in the leaf through which water could pass, such as 

 Sauvageau (1891, II, pp. 127 ff.) has described for Zostera, Phyllos- 

 padix,' Halodule and Potamogeton ; moreover, the fact that the 

 epidermal cells of the leaf, with the exception of the secretion cells, 

 are all photosynthetic and absorb solutions from the outside into 

 the interior, indicate the giving off of a very small amount of water, 

 if any from the leaves. Waste gases, however, can easily pass out 

 through the cell walls in solution. On account of these conditions, 

 structures adapted to the retention of a supply of water and 

 reduction of transpiration, are unnecessary, and we accordingly find 

 an absolute lack of such halophytic adaptations in Ruppia. 



B. The Adaptation to a Salt Water Environment 



In one particular, however, Ruppia shows a distinct halophytic 

 adaptation. This is exhibited in its power to withstand the plas- 

 molytic action of salt water. Ganong (1903) has found that the 

 root hairs of certain land halophytes possess specific abilities to 

 resist plasmolysis in various solutions of sea water, showing in this 

 way a greater or less halophytic adaptation. He says (1903, 

 pp. 353, 354), " I found a close correspondence between the halo- 

 philism of a plant and the power of its root hairs to resist plas- 

 molysis. This power has of course been gradually acquired, but 

 what its physical basis is I do not know ; though we shall probably 

 find that substances osmotically equivalent to the salt of the sea 

 water have been formed in the sap of the hairs." 



But Ruppia, and all of the allied salt water genera, such as 

 Zostera, Phyllospadix, &c., show an even greater adaptation than 



