Riccia nigrella DC.

Riccia nigrella DC. Fl. Fr. 6: 193. 1815,
Riccia minima I,. Sp. PI. 1139, in part. 1753. Riccia aggregata Underw. Bot. Gaz. 19: 275.
1894.
Thalli small to medium-sized, 2-8 mm. long, 1-4 times dichotomous, at first subradiate, later commonly irregularly gregarious, dark-green, somewhat polished and closely reticulate above, dark-purple or nearly black (rarely decolorate) beneath; main segments at first obovate, finally linear or linear-obcuneate, 0.75-1.5 mm. wide; terminal segments obcuneate-oblong or elliptic-obovate , obtuse or subacute; margins often brownish, naked, acute or slightly membranous, becoming rather obtuse in older parts, commonly erect-connivent on drying; median sulcus narrow and acute toward apex, becoming obtuse and finally obsolescent in older parts; scales blackish-piurple and nitent (rarely decolorate), transverse, semi-orbicular, not exceeding the margin; transverse sections subquadrate-semiorbicular in yoimger parts, subquadrate or oblong in the older, 1-2.5 (mostly 1.5-2) times as as broad as high, mostly 18-25 cells thick in median parts, the ventral outline almost rectilinear, slightly convex toward apex; dorsal epidermis unistratose, its cells lightly protuberant, subquadrate in vertical section, 25-55 /a broad, often broader than high, filled with a transparent and colorless or slightly grumous, somewhat refringent fluid (reacting strongly to iron-haematoxylin stain), the cells collapsed and disintegrated only in the oldest parts of the thallus. Monoicous; antheridial ostioles not at all or only very slightly elevated; capsules usually numerous, often crowded near bases of segments, the long intact, usually light-brown covering commonly with a purple spot about the exserted archegonium-neck; spores brown, becoming darker and more opaque with age, 60-90 ju in maximum diameter, angular, with a minutely granulate or nearly smooth, sometimes interrupted margin 3-6 ^ wide, the outer face irregularly areolate, or sinuous-ridged, often showing in profile obtuse or truncate papillae 2-4 p. long, the commonly imperfect areolae 3-10 ju wide, the inner faces rather more regularly areolate, with low, thickwalled, scarcely papillate meshes.
Type locality: Near Montpellier, France.
Distribution: Texas and California; reported by Austin from Pennsylvania and by Underwood from New York; also in Europe.