180 



MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



[yo/. 9, No. 2 



generously gave of. his time in the preparation of this manuscript as well as in 

 the evaluation of the botanical conclusions; Dr. B. Maguire, who suggested the 

 problem and set the writer's course; Dr. D. P. Rogers, whose knowledge of no- 

 menclatural problems was invaluable; Dr. H. W. Rickett for his assistance in 

 editorial and bibliographic matters; Miss Elizabeth Hall, whose help in the library 

 was appreciable; and Miss Lucille Kopp, who drew the included illustrations. 



GENERIC RELATIONSHIPS 



The Alismataceae are world-wide in distribution and quite varied in morphol- 

 ogy. All the included genera are characterized by having leaves with sheathing 

 bases; scapes with flowers in whorls, these subtended by a whorl of bracts; 

 hypogenous apocarpous flowers; seeds without endosperm, with campy lotropous 

 embryos. The perianth is composed of two whorls of three parts each; the outer 

 (sepals) persistent and herbaceous; the inner (petals) deciduous and white or 

 pink. 



The closest relationship exists between Sagittaria and Echinodorus, both of 

 which are characterized by numerous, spirally arranged, 1-celled, 1-ovuled achenes 

 on a dome-shaped receptacle. The former is predominantly temperate in distribu- 

 tion and has separate staminate and pistillate flowers, while the latter is mostly 

 tropical in distribution and has perfect flowers. This distinction is somewhat 

 weakened, however, by the presence of a ring of functional stamens on the pis- 

 tillate flowers of some of the more primitive Sagittarias; while the most primitive 

 species (S. guyanensis) occasionally shows signs of aborted carpels in the stam- 

 inate flowers. One other character, more or less constant, serves to separate the 

 two genera — the achenes of Sagittaria are much more flattened and compressed 

 than those of Echinodorus , 



Until recently the more primitive members of Sagittaria were always retained 

 in a separate genus Lophotocarpus. These predominantly tropical or warm- 

 temperate plants do show some gradations between Echinodorus and Sagittaria as 

 discussed above; however, their mergence with the subg. Sagittaria is complete. 

 The chief arguments of those who would maintain Lophotocarpus as a separate 

 genus are based on the presumption that the Lophotocarpi are annual with stam- 

 inate flowers above and perfect flowers below, while the Sagittariae are perennial 

 with separate staminate and pistillate flowers. The first premise is no more than 

 a vagary of edaphy and climate; both groups contain species that are facultatively 

 perennial or annual according to their habitats. The second premise would seem 

 to be a case of poor observation, since nearly all species in both groups may 

 demonstrate perfect flowers occasionally, while, even more frequently, species of 

 Lophotocarpus may have separate pistillate and staminate flowers. Lophotocarpus 

 therefore cannot be maintained on a generic level. It is preserved here as a 

 subgenus by combining with it several species of Sagittaria that demonstrate the 

 same characteristics of reflexed, thickened, pistillate pedicels, appressed to 

 spreading pistillate sepals, and occasionally perfect flowers. The reasons for 

 not maintaining Lophotocarpus as a separate genus are capably reviewed by 

 Mason (1952). 



HISTORY OF THE GENUS 



Linnaeus in his Species plantarum (1753» p. 993) established Sagittaria as a 

 genus, basing the name on numerous publications of earlier writers. He included 

 therein three species, of which only one, 5. sagittifolia t has been retained. Of 

 the others, S. obtusifolia has since been transferred to Limnophyton, while 5. 



