66 A. H. Graves, 



A few remarks may be in order here as to the Uterature which 

 relates to the subject of this paper, and which I have found most 

 helpful in its preparation. Ascherson (1889), in Engler and Prantl's 

 Natiirliche Pflanzenfamilien, cites the most important references 

 bearing on the Potamogetonaceae up to the date of his contribution. 

 Among these, the papers on Althenia by Prillieux (1864), Cymodocea 

 by Bornet (1864), and Zostera by Gronland (1851) are valuable 

 for their contributions to the morphology of these allied plants. 

 In this respect, also, the works of Irmisch (1851 and 1858) are 

 remarkable for the accuracy with which they describe the external 

 morphological relations of R. rostellata and other Potamogetonaceae. 

 Of later works, which are especially useful in a comparative mor- 

 phological study of the vegetative organs, are, among others, those 

 of Sauvageau (1891, II), Campbell (1897) and Goebel (1898). As 

 to ecological considerations pertaining to water plants, the work of 

 Schenck (1886) deserves especial mention. In the preparation of 

 the parts on the reproductive organs, embryo, &c., the works cited 

 are tdo numerous to mention here, and the reader is referred to the 

 bibliography (p. 165). 



If we except a rather doubtful reference of Hofmeister, (1861, 

 Figures 1—7, PI. II, and see p. 148 of this paper), the only investigation 

 of Ruppia maritima which has ever been published, so far as I can 

 ascertain, is that of Roze (1894), who describes chiefly the con- 

 ditions of fertilization, but also presents a valuable historical review 

 of the whole genus. Others, it is true, such as Miss Scott (1906) 

 and Chrysler (1907), have dealt with special features of the plant, 

 in the course of their comparative morphological investigations. 



Ruppta rostellata, on the other hand, if we piece together the 

 results of various observers, has been pretty thoroughly worked out. 

 First Irmisch (1851 and 1858) described carefully its external mor- 

 phological characters ; next, Wille (1883) studied the development 

 of the embryo, and quite recently Murbeck (1902) published an 

 admirable paper on the reproductive organs and embryo. 



The genus Ruppia is aquatic, characterized by its long, linear, 

 grass-like leaves with basal sheaths, and grows in brackish and salt 

 water (but probably never salt water of normal ocean strength, 

 see p. 124), in creeks and bays along the coast and in the neigh- 

 borhood of inland salt springs, throughout all parts of the temperate 

 and tropical zones. Except at low tide it is completely submerged 

 until the period of flowering, when it produces its flowers a short 

 distance (1—3 cm.) above the surface of the water. The flowers, 

 always two to each peduncle, are borne one above the other, on 



