1955] 



REVISION OF THE GENUS SAGITTARIA 



183 



depth of the water in which they begin growing; however, it seems that once a 

 leaf does respond to a given depth of water in its initial growth, there can be no 

 further response in that particular leaf no matter how its environment changes. 

 Submerged leaves are always bladeless and are considered phyllodia. They may 

 be strap-shaped or terete, and their presence and shape are often useful tools 

 in identifying plants (i.e., the spongy, terete phyllodia of S. graminea var. teres). 

 In tidal forms such as S. montevidensis subsp. spongiosa the phyllodia often have 

 somewhat broadened or spatulate ends. The floating leaves regularly form linear 

 to ovate blades with bases occasionally cordate. The floating cordate blades of 

 S. cuneata, when present, and of 5. guyanensis are nearly certain guides to the 

 identity of these species. The emersed leaves have linear to ovate blades which 

 may be variously sagittate. These, in correlation with other factors, while almost 

 never of specific value, are useful for delimiting various groups of species. The 

 one exception is S. longiloba, easily recognized by the basal lobes of the sagit- 

 tate leaves which are always longer than, and mostly twice as long as, the blade. 

 The venation of the leaves has often served as a key characteristic for many au- 

 thors; however, it is a most variable factor that is rarely satisfactory for identifi- 

 cation except in the sagittate species of the subgenus Lophotocarpus (5. sprucei, 

 S. montevidensis and S. intermedia), which have characteristic prominent?- veined 

 leaves. 



Much discussion and speculation have centered on which of the various 

 leaf-forms are primitive. Arber (1920, p. 22, 23) reviews this thoroughly and con- 

 cludes that the phyllodial leaves, which are always the first formed no matter 

 what the environment, are primitive, while the sagittate are advanced. 



Pubescence. The presence of pubescence on vegetative parts has been re- 

 ported in only two taxa, S. guyanensis and S. latifolia var. pubescens. Its presence 

 is therefore diagnostic. Various floral parts may be pubescent, such as bracts, 

 sepals, and filaments. The significance of pubescence on these parts is discussed 

 below. 



Inflorescence. The scapes of Sagittaria are emersed and erect or lax and 

 floating. They rarely furnish a key to specific identity although many works stress 

 the ratio between leaf- and scape-length as an important feature. The exception 

 is the unique geniculate scape of 5". rigida. The Sagittaria inflorescence is an 

 interrupted spike with 1-12 whorls of flowers. Occasionally the lower flower- 

 whorls are replaced by branches and the scape has a panicled appearance. 



The whorls are typically, although not always, 3-f lowered, and each whorl is 

 almost always subtended by a ring of 3 bracts. These bracts have been neglected 

 by most workers in the genus, but the present writer has found extremely useful 

 characters in them to distinguish taxa of all grades. Their texture (membranous or 

 firm), indument (glabrous, pubescent), corrugations (papillose, striate), degree of 

 basal fusion (free, joined, or connate), size, and shape are often the only sure 

 guides to identity in the absence of mature achenes and workable stamens. 



The pedicels also offer useful characters. They may be ascending, divaricate, 

 or recurved, and variously thickened. Those of the pistillate flowers in combina- 

 tion with certain characteristics of the sepals furnish the basis for the separation 

 of the genus into two rather well-marked subgenera (figs. 2e, 17d). 



Flowers. The flowers of Sagittaria are usually unisexual with the staminate 

 above and the pistillate below on the same scape. Individuals of several species 

 may occasionally be unisexual but monoecious plants are the rule. Hermaphrodite 

 flowers occasionally occur, sporadically in most species but characteristically in 



