Rhizomes with proximal cordlike portion 0.8-2 dm × 2 mm, bearing brown, scalelike leaves and rarely roots, distal portion enlarging to 3-5 × 0.8-1.2 cm, producing fibrous roots, 2-3 brown, sheathing, bractlike leaves, then 8-12 green foliage leaves; usually 2 cordlike rhizomes produced from apex as new growth begins in spring. Stems simple, 0.8-4 cm. Leaves: basal with blade broadly linear at anthesis, slightly falcate, 4-6 cm × 6-8 mm, enlarging to 16 cm × 10 mm; cauline with proximal 1-2 resembling basal leaves, 4-6 cm, distal 2 more sheathing of stem, exceeding spathes. Inflorescence units 1(-2)-flowered; spathes green, outer closely sheathing inner and enclosing ovary, slightly keeled at midrib, 4.5 cm, ± equal, margins scarious. Flowers: perianth sky blue; floral tube dull yellow, funnelform, 1-2 cm, dilated upward to 4-5 mm diam.; sepals 2-2.3 × 0.8 cm, tapering gradually into claw, apex emarginate, signal white with deep violet margin, with 3 yellow and white, toothed, low ridges; petals spreading, narrowly oblanceolate, 1-1.5 × 0.4-0.5 cm, shorter and narrower than sepals, apex emarginate; ovary sharply trigonal, 0.8-1 cm; style 1-2 cm, crests linear to semiovate, 4 mm, margins crenate; stigmas rounded, margins entire; pedicel 0.6-1.5 cm. Capsules roundly triangular, 1.2 × 8 mm, enclosed in spathes. Seeds dark brown, with white appendage spiraled around seed, ca. 3 mm, quickly drying when exposed to air. 2n = 32, 42. Flowering May. Moist, sandy gravel, limestone shores of lakes; Ont.; Mich., Wis. Several authors have described Iris lacustris simply as being very similar to I. cristata, but smaller in every detail. W. R. Dykes (1913) said, 'This name may be retained as that of a local form or variety but it hardly deserves specific rank.' He treated the taxon as I. cristata var. lacustris. R. C. Foster (1937) discussed the relationship between I. cristata and I. lacustris in detail, and recognized the latter at specific rank. He stated, 'It is interesting to note that it has been recorded in several localities on or near the Bruce Peninsula, Ontario, that home of so many pre-Wisconsin relics. Professor Fernald informs me that he has seen it growing there in some abundance on the upper levels of the sandy beaches. Quite probably, it is present there, not as a newcomer, but as a relic.' However, G. L. Hannan and M. W. Orick (2000) concluded that I. lacustris is a product of some post-glacial invasion or introduction of a plant or seed of I. cristata that has developed its own very stable and quite unvariable character.
Like a smaller form of no. 2 [Iris cristata Aiton]; lvs broadly linear, curved- arching, 4-6 cm at anthesis, later to 18 cm נ5-10 mm; spathe-valves closely sheathing each other, the upper and lower equally spaced; perianth-tube dull yellow, 1-2 cm, dilated upwards, not or scarcely exceeding the spathe-valves and shorter than the sep and pet; fls 5-6 cm wide, the pet emarginate; seeds dark brown, arillate; 2n=42. Gravelly shores and cliffs in calcareous soil around lakes Michigan and Huron; now rare. May.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.