66 A. H. Graves, 
A few remarks may be in order here as to the literature 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 Grénland (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 FR. roste/lata 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 too 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, Pl. 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. 
Ruppia 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 

