116 A. H. Graves, 



a halophyte. And when one reviews the strikingly diverse char- 

 acters exhibited b}- plants living in salty soil — which is the vegetation 

 one naturally thinks of as halophytic— and by those living in salt 

 water, it seems best to avail ourselves of Schimper's (1898, p. 817) 

 terms " land halophyte " and " water halophyte " as a solution of the 

 difficulty. 



Considered as a water halophyte, therefore, Ruppia has both 

 hydrophytic and halophytic adaptations. I shall outline first the 

 former, taking up in order those modifications that occur in the 

 shoot and in the root, and secondl}' describe the halophytic adaptations. 



Hydrophytic Adaptations 

 A. The Shoot 



1. Gross Morphological Adaptations. 



In its external form and style of branching the stem of Ruppia 

 presents a marked contrast to the stem of a typical land plant. 

 Since water has such a greater density than air, it becomes no 

 longer essential for the plant to have a stout, firm axis capable of 

 holding itself erect and supporting numerous branches. Instead we 

 find the stems of Ruppia long and slim, and except at the very 

 apex, of equal diameter throughout. For the same reason the 

 branches are similar, and as long or longer than the principal axis 

 from which they arise. Hence the peculiar wide-spreading branch 

 system which I have already described — a type impossible in a 

 land plant without a copious development of strengthening tissue. 



The leaves of Ruppia, arising from such a weak, slender stem, 

 are of enormous length compared with the leaves of an ordinar}^ 

 land plant having a main axis of similar diameter. This extreme 

 length is again made possible by the greater density of the sur- 

 rounding medium. A leaf of similar proportions in a land plant 

 would have great difficulty in holding itself out in a plane suitable 

 for receiving the rays of light. 



The advantages of this extreme length seem quite apparent. 

 Coupled with the narrowness of the leaf, the effect is just as in 

 aquatics with finel}- divided leaves, such as Batrachium, Cerato- 

 phyllum, &c., i. e., to present a large amount of surface to the 

 water in proportion to the volume of the leaf. This increase in 

 leaf surface is beneficial for several reasons. In all probability much 

 of the nutrient mineral solution requisite for metabolism is absorbed 

 b}- water plants directly from the surrounding water. Moreover, 



