ENGELMANN THE GENUS ISOETES IN N. AMERICA. [377 



§ 0. Enumeration of the Species. 



I. Trunk bilobed. 



A. Submerged, normally growing under water, only in unusually dry seasons 

 coming above the surface; leaves quadrangular, without peripheral bast- 

 bundles; velum incomplete. 

 * Without stomata. 



I. I. LACUSTRis, Lin. Leaves stout, rather rigid, obtusely quadrangu- 

 lar, acute but scarcely tapering, dark or olive-green, 10 to 25 in number, 

 2 to 6 inches long; sporangium orbicular to broadly elliptical, not spot- 

 ted, with a rather narrow velum; ligula triangular, short or somewhat 

 elongated ; macrospores 0.50 to 0.80 mm.* in diameter, marked all over 

 with distinct or somewhat confluent crests; microspores smooth, 0.035 to 

 0.046 mm. in the longer diameter. — Syst. Veg. I. 1753; Durieu Bull. Bot. 

 Soc. France, 8, p. 164, 1861 ; Gray Man. ed. 5, p. 675. 



Van PAUPERCULA with fewer (10 to 18), thinner, shorter (2 to 3 inches) 

 leaves and smaller spores (macrospores 0.50 to o 66 mm, diam. ; micro- 

 spores somewhat granulated, 0.026 to 0.036 mm. long). 



A northern species of Europe and America, generally gregarious on 

 gravelly soil in the bottom of lakes under i to 4 or 5 feet of water, farther 

 south only on mountains; maturing in Sept. and Oct. Catskill Moun- 

 tains, N. Y. Sch-weinitz, Echo Lake, Franconia Mountains, N. H. Tuck- 

 ertnann, E7igclniann : in Massachusetts, in Fresh Pond near Cambridge, 

 W. Boott and Uxbridge, J. W. Bobbins; Brattleborough, Vt., C. C.Frost; 

 Saulte de Ste. Marie on Lake Superior. Porter and Leidy. The variety 

 in Grand Lake, Middle Park, Colorado, over 8,000 ft. alt., Engelmann, 

 and in Castle Lake near Mt. Shasta, California, 7,000 ft. alt., C. G. Prin- 

 gle. — This is the original Linnean species, fo-merly confounded with 

 others, and first clearly established by Durieu, 1. c. It is always readily 

 recognized by its rigid, rather thick, not gradually tapering dark green 

 leaves, which do not collapse when taken out of the water, and by the size 

 and sculpture of the spores. The variety pauperctila is based on western 

 mountain specimens, and is characterized by the smaller proportions of 

 all parts, and especially of the (for the species) unusually small micro- 

 spores. Durieu, I.e. 11, p. loi, distinguished a form with exceptionally 

 large macrospores (0.70 to 0.80 mm. diam.) as /. macrospora from a sin- 

 gle specimen from the Herb. Acad. N. S. Philad., with the label "Catskill 

 Mountains" in the handwriting of Schweinitz; but others show sometimes 

 spores of similar dimensions, e.g. specimens from Lake Superior; and 

 such have also been found in Europe, though there the spores rarely reach 

 a size of over o 65 to 0.70 mm. 



2. L PYGM^A, Engelm. One of the smallest species with a few (5 to 

 10) short {\ to I inch long) stout, rigid, bright green leaves, abruptly 



* I adopt for the smaller measurements the metrical system, which will gradually but 

 surely supersede the old and clumsy method, while in the larger measurements, as the 

 length of leaves, I still adhere to the foot and inch as the one yet best understood. The 

 millimeter is, as is well known, equal to very nearly half a line. 



