This globally rare rhizomatous diploid herb (2n = 18) is widely but thinly scattered from the central Atlantic Coastal Plain to the central Mississippi Valley, but largely restricted to somewhat open marshy or boggy habitats (W). The species has been overlooked in some regions, and it deserves further search in the field and the herbarium (Y, W). H. adpressum is currently listed as threatened, endangered or locally extinct in all states (NS). There are several records from a north-south zone across c. Tenn. (Ch), suggesting dispersal along a flight path of waterfowl. It has been reported from Ky. based on colls. of R. Athey made during 1968-70 in BALL (including #225 at MUR, #230 at MEM and #307 at TENN) and MCRA (including #178 at MUR, #254 at MEM and NCU, #262 at EKY). However, most or all of these appear to be misidentifications: #254 = ?sphaerocarpon (but relatively broad leaves); #262 = ?punctatum; #307 = ?lobocarum. Athey's coll. #254 (from Bloom Avenue near railroad overpass) resembles the broad leaved Ozarkian segregate of sphaerocarpon; see notes under that species. H. adpressum appears closest to nudiflorum, but differs as follows (Sm, F, Y, W): stamens persistent (F), "forming a marcescent mass at base of capsule" (versus "promptly deciduous, the capsules naked at base"); sepals mostly 3-7 mm long (versus 3-4 mm), acute to acuminate (versus obtuse to subacute); plants strictly herbaceous (versus usually subshrubby), usually 3-8 dm tall (versus 5- 20 dm). The whole adpressum-nudiflorum-apocynifolium complex appears close to sphaerocarpon (or its potential segregates), but reportedly differs as follows (Sm, F, Y, W): capsules ellipsoid to ovoid (versus broadly ovoid to subglobose), incompletely 3-celled by intrusion of the placentae (versus 1-celled with parietal placentae), producing numerous seeds (versus 4-8), 0.6-1.8 mm long (versus 2-2.7 mm); leaves 5-30 mm wide (versus 3-15 mm), with cuneate to truncate base (versus truncate to clasping).