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 entirely 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 (1894, II, 
p. 121) speaks of Althenia filiformis and A. Barrandonii, which 
grow side by side in certain ponds near Montpellier: “Les feuilles 
de la premiere ne possedent jamais d’elements épaissis; celles de 
la seconde, au contraire, ont non seulement leur unique nervure 
entouree d’un endoderme puissant, mais l’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 l’on admet l’action 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 “l’etat anatomique .... maintenant depend 
assurement non seulement du temps depuis lequel V’adaptation a 
commence, mais aussi de leur structure originelle et de leur reési- 
stance spécifique a l’adaptation, autrement dit, des caracteres qui 
leur ont ete legués 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 they 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 highly adapted plants as Ceratophyllum 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. 
