126 A. H. Graves, 



Adaptation and Heredity 



When one reviews the manifold ways in which Ruppia is adapted 

 to its environment, it becomes clear that this plant represents an 

 advanced stage of special evolution, resulting in a particular type 

 of plant, growing entireh' below the surface of the water and almost 

 ideally adapted to the novel conditions which present themselves. 



That the characters of aquatic plants cannot, however, always 

 be explained on the hypothesis of adaptation, Sauvageau has well 

 illustrated. Another factor should be reckoned with, namely, he- 

 redity. For instance, among other examples, Sauvageau (1891, 11, 

 p. 121) speaks of Althenia filiformts and A. Barrandonii, which 

 grow side b}' side in certain ponds near Montpellier : " Les feuilles 

 de la premiere ne possedent jamais d'elements epaissis ; celles de 

 la seconde, au contraire, ont non seulement leur unique nervure 

 entouree d'un endoderme puissant, mais I'epiderme a ses parois plus 

 epaisses, le limbe est parcouru par deux gros cordons fibreux plus 

 ou moins lignifies, et la gaine en montre plusieurs semblables. Ces 

 faits sont completement inexplicables si Ton admet Taction predo- 

 minante et quasi exclusive du milieu." 



It is evident that as Schenck (1886, p. 7) has declared, adaptation 

 and heredity are two opposing factors in the transformation of an 

 organism, and that " I'etat anatomique .... maintenant depend 

 assurement non seulement du temps depuis lequel I'adaptation a 

 commence, mais aussi de leur structure originelle et de leur resi- 

 stance specifique a I'adaptation, autrement dit, des caracteres qui 

 leur ont ete legues par heredite." (Sauvageau, 1891, II, p. 120.) 



One meets in Ruppia very few of these useless structures handed 

 down presumably from former generations. We have seen that the 

 cortical bundles of the stem are evidently rudimentary structures, 

 but one cannot affirm with certainty that the}' are now useless. 

 Again, the tracheae of the axial bundle in the shoot become so 

 disorganized in the mature condition that it would seem as if here 

 were an ancestral structure which is no longer needed. And, as 

 if to carry out this idea, we find the tracheae absolutely lacking in 

 such highh' adapted plants as Ceratopb3'llum and Naias (Schenck, 

 1886, p. 30). 



Summary 



1. It is best to classify Ruppia ecologically as a water halophyte. 

 As such it exhibits both hydrophytic and halophytic adaptations. 



