700 SELAGINELLACE^. 



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. (SetcheU), 

 and northward. — Distinguished from the nearly allied I. lacustris by the sto- 

 mata of the leaves, the spotted sporangium, the smaller size of the macrospores 

 and their reticulation on the lower half. 



6. I. Engelmanni, 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. gracilis, 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., SetcheU) and N. J. [Ennis) ; entirely submersed! 



Var. valida, Engelm. Trunk large and stout (often 1-2' wide); leaves 

 (50-100, even 200, 18-25' long) with an elevated ridge on the upper side; 

 sporangium oblong or linear-oblong (4 -9" long), J -^ or more covered by 

 the velum; spores very small; macrospores 0.16-0.22" wide; microspores 

 0.01 1 - 0.013" long, spinulose. — Del. ( Canby) and Penn. [Porter). Sept. 



7. I. melanopoda, J. Gay. Leaves (15-50, 6-10' long) very slender, 

 keeled on the back, straight, bright green, usually with dark brown or black 

 shining bases ; sporangium mostly oblong, with a very narrow velum, brown 

 or spotted ; macrospores very small (0.14 - 0.1 8" wide), smoothish, or with faint 

 tubercles or ridges; microspores (0.010-0.012" long) spinulose. — Shallow 

 ponds, and wet prairies and fields, central and northern 111. (E. Hall, Vasey), 

 and westward. June, and sometimes again in Nov. — Trunk more spherical 

 and more deeply 2-lobed, and both kinds of spores smaller than in any other of 

 our species ; leaves disappearing during the summer heat. Closely approach- 

 ing the completely terrestrial species of the Mediterranean region. 



Order 135. MARSILIACE^. 



Perennial plants rooted in mud, having a slender creeping rhizome 

 and either filiform or 4-parted long-petioled leaves ; the somewhat crus- 

 taceous several-celled sporocarps borne on peduncles which rise from the 

 rhizome near the leaf-stalks, or are more or less consolidated with the 

 latter, and contain both macrospores and microspores. 



1. MARSILIA, L. (PI. 25.) 



Submersed or emersed aquatic plants, with slender creeping rootstocks, 

 Bending up elongated petioles, which bear at the apex a whorl of four nervose- 

 Teined leaflets, and at or near their base, or sometimes on the rootstock, one 



