Wheldon : Key to the Harpidioid Hypna. 
15 
alone lightly secund inclined : small, slender, fawn coloured and slightly 
branched, branches irregular, short (W . fluitans var. falcata f. shet- 
landica). 
26. Leaves 2 mm. or more long, feebly falcate : plant larger, 5-10 
cm. high {W . fluitans var. ovale). Leaves 1*5 mm. long or less, com- 
paratively wider and shorter : plant small and slender with strongly 
secund -falcate leaves {W.' fluitans var. ovale f. densum) . 
27. Nerve comparatively weak (85-100 /a on well developed stems) : 
plant paler (glossy greenish-yellow to pale brown, darker below) : 
leaves denticulate all round : auricles often rounded and projecting : 
texture delicate, median cells thin walled, translucent {W . fluitans 
var, Arnellii ). Nerve stronger (100-130 ja ) , plant more deeply coloured : 
auricles less prominent : leaf texture more solid (28). 
28. Leaves longly subulate : cell walls moderately incrassate (29). 
Leaves shortly subulate : cells with strongly thickened, coloured and 
interrupted walls : plant stout, 15-20 cm. high, densely pinnate, brown, 
-soon purplish, then blackish (W . fluitans var. procera). 
29. Dull yellowish-green, fuliginose below : leaves denticulate, erect- 
patent, weakly falciform at stem apex : pinnate above, pinnae ascending : 
nerve 80-130 /jl (W . fluitans var. anglica) . Greenish, soonpurple, passing 
to brownish -black : leaves subentire, falciform : tall (20-25 cm.) and 
negularly pinnate : nerve 100-120 7* (W. fluitans var. tricolor). 
30. Auricles of many thin-walled polygonal cells disposed in, 3 or 4 
tows (31). Auricles of fewer longer rectangular curved cells disposed in 
1 or 2 rows, sometimes incrassate and coloured (39). 
31. Acumen short and wide : supra -basal cells short and wide (32). 
Acumen longer and narrower, as also the cells (35). 
32. Plant arctic : leaves small (i-|-2 mm.), acumen very short, 
lingu late -obtuse at times subcucullate : apical cells dilated, rhomboid 
(W . exannulata var. tundrae). Plant montane or of the plains, without 
above characters (33). 
33. Robust : leaves large, widely oblong oval, suddenly shortly 
cuspidato -acuminate, erect appressed, closely imbricate at apices of 
stem : nerve stout, 90-140 /a at base (W. exannulata var. orthophylla) . 
Slender : leaves less suddenly acuminate : nerve narrower, 50-90 /a (34). 
34. More or less pinnate and elongate : leaves (at least the upper 
ones) falcate -secund : nerve (66-90 /a) wide (W. exannulata var. hr achy - 
dictyon) . Slightly branched, short, glossy : upper leaves erect appressed, 
obtuse, giving the appearance of Calliergon stramineum or Acrocladium 
cuspidatum : nerve 50-60 ^ wide (W . exannulata var. Nicholsoni) . 
35. Leaves oval to oval-oblong acuminate, falcate -secund (36). 
Leaves from a narrower oblong lanceolate base, more longly acuminate, 
flexuos,e below, falcate -secund above : basal cells longer and also cells 
frequently approaching in structure those of W. stenophylla (W. exannulata 
var. pinnata f. stenophylloides) . 
36. Leaves falcate recurved (37). Leaves almost falcate circinate 
above, nerve very strong (38). 
37. Erect, green, often tall and pinnate, nerve 90-108 /a (W . ex- 
annulata var. pinnata f. acuta). Procumbent, irreg. branched or nearly 
.simple, more slender, nerve 66-70 /a {W . exannulata var. pinnata f. 
gracilescens) . Robust and often blotched with purple (W . exannulata 
var. pinnata f. versicolor) . 
38. In large depressed patches, stems much divided, divisions 
irregularly pinnate, dark brown to blackish -brown below : leaves strongly 
Ramato -secund, approaching in habit L. revolvens (W . exannulata var. 
pinnata f. montana). In large swelling compact silky cushions, golden 
.green above, ofhnge-brown below : stems repeatedly divided and pro- 
liferous, divisions with crowded fastigiate branches : leaves very dense, 
appressed below, subcircinate above (W . exannulata var. pinnata f. 
polyclada) . 
1922 Jan. 1 
