Wheldon : Key to the Harpidioid Hypna. 
11. Dull green above, reddish-brown below : upper leaves secund- 
falcate, lower complanate subpatent : auricles scarcely decurrent (W. 
jluitans var. squalida). Pale green, stained vinous-red below : leaves 
straight, erect patent : auricles distinctly decurrent (W. fluitans var. 
Payoti). ' 
12. Glossy slender occasionally purple stained : leaves small (under 
3 mm.), densely imbricate, points only recurved : nerve short, scarcely 
•exceeding mid-leaf : cell walls solid slightly incrassate and porose at 
leaf base (W . fluitans var. Robertsiae) . Yellowish-green passing to 
red and brown : leaves larger (5-7 mm.), nerve reaching £ : basal cells 
less incrassate, not porose (W . fluitans var. setijorme). 
13. Stems filiform, subsimple : leaves distant horizontally patent : 
.auricles small and obscure (W .< fluitans var. Jeanbernati f. tenella) . 
Stouter, leaves not patent (14). 
14. Leaves entire, erect, oblong, shortly acuminate : upper cells 
rather short, supra-basal ones lax and parenchymatous, alar cells a little 
dilated (W . fluitans var. pseudostraminea) . Leaves more or less serrate, 
at least near apex (15). 
15. Auricles nearly or quite obsolete, or if distinct, nerve short 
and often furcate (16). Auricles more distinct (18). 
16. Tall and robust, pinnate : leaves strongly falcate throughout, 
resembling a rather robust D. Sendtneri : auricles obsolete (W . fluitans 
var. scotica). Smaller, not pinnate : leaves feebly falcate at apex only, 
or not at all (17). 
17. Leaves distant, flexuose, finely acuminate, thinly nerved half 
way : nerve occ. furcate (W . fluitans var. Jeanbernati f. Holleri). Very 
small (1-3 cm.), leaves lightly homotropous, acumen short and rather 
obtuse, toothed : nerve short, often furcate, in rameal leaves nearly 
obsolete (W. fluitans var. hemineuron ) . 
18. Auricles strongly decurrent, supra-basal cells lax: nerve 35-60 /a, 
plant submerged, elongate, pale green, with large (4-7 mm.) lax, flexuose 
or feebly falciform leaves, often patent in lower part of stems (W . fluitans 
var. submersa). Auricles hardly decurrent, cells nearly uniform to 
.base : leaves closer, lower usually erect or subsecund (19). 
19. Cells very long and narrow : nerve slender (46-50 /u) : dull 
.green (20). Cells rather shorter and wider, more chlorophyllose : nerve 
a little stronger (52-64 fi) : bright glossy green (W. fluitans var. 
atlantica) . 
20. Leaves rather large and wide : plant tall (15-25 cm.), much 
divided, divisions pinnately ramulose (W . fluitans f. elata). Leaves 
smaller and narrower : plant (5-15 cm.) feebly branched, scarcely 
pinnate : nerve 46-48 /i at base (W . fluitans var. Jeanbernati). 
21. Nerve comparatively weak (34-90 ju) at base : plants usually 
slender and rather short (22). Nerve stronger (90-120 /u) : plants 
typically robust and pinnate (27). 
22. Very long but slender (15-25 cm.), more or less pinnate, leaves 
of branches plumosely spreading : leaves narrowly lanceolate, nerve 
weak (40-60 /jl) and short, hardly exceeding middle of leaf : basal cells 
coloured, in one or two rows reaching nerve (W . fluitans var. Delamarei) . 
Shorter (5-15 cm.), rarely pinnate, often subsimple (23). 
23. Leaves lax, narrowly acuminate, not, or only shortly, subulate 
(24). Leaves dense, longer, grad, acuminate, ending in a long subulate 
point ; greenish -yellow above, deep chestnut below (W . fluitans var. 
Jalcata f. alpina). 
24. Leaves grad, acuminate, from an oblong lanceolate to lanceolate 
base: nerve 50-60 [a wide (25). Upper leaves rather suddenly acuminate 
from an oval base : nerve short and weak (33-50 /u), sometimes furcate 
above (26). 
25. Leaves regularly falcate-secund (W . fluitans var. Jalcata ). 
Leaves erect, lightly appressed, hardly falcate-secund, the leaf points 
Naturalist 
