3. I. echinóspora, Durieu. Leaves slender, awl-shaped; sporangium ovoid or circular; macrospores (0.20–0.25´´ wide) beset all over with small entire and obtuse or slightly forked spinules. (Eu.)—In this European form, the leaves are very slenderly attenuated (3–4´ long), the upper margin of the sporangium only is covered with the narrow velum, the free part is unspotted, and the slightly papillose microspores are larger (0.015–0.016´´ long).
Var. Braùnii, Engelm. Leaves (15–30 in number, 3–6´ long) dark and often olive-green, straight or commonly recurved, half or two thirds of the sporangium covered by the velum, the free part often with light brown spots; macrospores as in the type; microspores smaller (0.013–0.014´´ long), smooth. (I. Braunii, Durieu.)—Ponds and lakes, New Eng. to N. Y., Penn., Mich., and northward, often with the two preceding.—Frequently with a few stomata, especially in Niagara specimens.
Var. robústa, Engelm. Stouter; leaves (25–70, 5–8´ long) with abundant stomata all over their surface; velum covering about one half of the large spotted sporangium; macrospores 0.18–0.27´´ wide.—Lake Champlain, north end of Isle La Motte (Pringle).
Var. muricàta, Engelm. Leaves (15–30, 6–10´ long) straight or flaccid, bright green; about one half of the almost circular sporangium covered by the velum, unspotted; macrospores (0.22–0.27´´ wide) with shorter and blunter spinules; microspores as in the last variety, or rarely spinulose. (I. muricata, Durieu.)—In some ponds north of Boston (W. Boott).
Var. Boóttii, Engelm. Leaves (12–20, 4–5´ high) awl shaped, stiffly erect, bright green, with stomata; sporangium as in the last; macrospores as in the type, but a little smaller and with very slender spinules. (I. Boottii, Braun, in litt.)—Pond in Woburn, near Boston, partly out of water (W. Boott).
[*][*] Growing partly out of water, either by the pond drying up or by the receding of the ebb tide; leaves with stomata, and in n. 6 and 7 with four or more peripherical bast-bundles.
4. I. saccharàta, Engelm. Leaves (10–15, 2–3´ long) slender, olive-green, curved; sporangium small, ovoid, only the upper edge covered by the velum, nearly unspotted; macrospores (0.20–0.22´´ wide) minutely tuberculate; microspores (0.012´´ long) papillose.—On Wicomico and Nanticoke Rivers, eastern shore of Maryland, between high and low tide (Canby).
5. I. ripària, Engelm. Leaves (15–30, 4–8´ long) slender, deep green, erect; sporangium mostly oblong, upper margin to one third covered by the velum, the free part spotted; macrospores very variable in size (0.22–0.30´´ wide), the upper segments covered by short crested ridges, which on the lower hemisphere run together forming a network; microspores larger than in any other species except n. 1 (0.013–0.016´´ long), mostly somewhat tuberculated.—Gravelly banks of the Delaware, from above Philadelphia to Wilmington, between flood and ebb tide; margins of ponds, Lake Saltonstall, Conn. (Setchell), and northward.—Distinguished from the nearly allied I. lacustris by the stomata of the leaves, the spotted sporangium, the smaller size of the macrospores and their reticulation on the lower half.
6. I. Engelmánni, Braun. Leaves long (25–100, 9–20´ long), light green, erect or at last prostrate, flat on the upper side; sporangium mostly oblong, unspotted, the velum very narrow; macrospores (0.19–0.24´´ wide) covered all over with a coarse honeycomb-like network; microspores (0.012–0.014´´ long) mostly smooth.—Shallow ponds and ditches, from Mass. (near Boston, W. Boott, H. Mann) and Meriden, Conn. (F. W. Hall), to Penn. and Del. and (probably through the Middle States) to Mo.—By far the largest of our species, often mature in July.
Var. grácilis, Engelm. Leaves few (8–12 only, 9–12´ long) and very slender; both kinds of spores nearly as in the type.—Southern New Eng. (Westville, Conn., Setchell) and N. J. (Ennis); entirely submersed!