Monographs Details: Rhynchospora pilosa (Kunth) Boeckeler
Authority: Maguire, Bassett. 1972. The botany of the Guayana Highland--part IX. Mem. New York Bot. Gard. 23: 1-832.
Discussion:This species is easily discernible in the stiff hairy leaves and culms and in the large pale-greenish head. Uittien described R. arenicola from Suriname separating it from R. pilosa by “die geringere Behaarung und die grösseren und bleichen Ahrchen.” Examination of ample collection from the Guayana region shows that this segregation is quite weak. The “sparse pubescence” in R. arenicola can hardly be held since several Guayanan specimens bear quite heavily pubescent leaves and culms, while nearly glabrous leaves and culms were seen in William 6909 (from Minas Gerais, Brazil), which otherwise falls under the category of R. pilosa. The “larger spikelets” in R. arenicola appear to be somehow pertinent in northern South American plants, but with occasional exceptions, for instance, Sandwith 1423 from British Guiana, in which the spikelets are only 5.5 to 6 mm and thus make a better match with those of R. pilosa. My own observation shows that there are small differences in achenes between R. arenicola and R. pilosa. In the former, achenes are slightly larger than those of the latter at 1.3 to 1.5 vs 1.0 to 1.2 mm in length, and the conical style-base tends to be longer than the depressed one in the latter. In addition to these floral characters, R. arenicola is generally more robust than R. pilosa. In other characters, especially in the shape and surface pattern of achenes, the two entities well agree with one another. Nevertheless, the minor differences mentioned above are correlated to the geographical separation. Namely, the range of R. arenicola lies north of Amazonian Basin from Brazilian Amapá through Suriname and Guiana westwards to Venezuela Amazonas, while R. pilosa occurs in southeastern Brazil and from Bahia southwards to Minas Gerais and to Parana. On this basis I propose to treat R. arenicola as the subspecies of R. pilosa.