DUCKWEED FAMILY. 367 
5. Lemna gibba L. Gibbous Duckweed. 
(Fig. 889.) 
Lemna gibba I. Sp. Pl. 970. 1753- 
Thallus slightly unsymmetrical, obovate or short-obovate, 
114//-3’’ long, thickish or more or less strongly gibbous be- 
neath, short-stalked when young, soon separating, provided 
with stomata which are sparse beneath, obscurely 3-5-nerved; 
rootcap mostly short-pointed, rarely long-pointed or obtuse; 
fruit symmetrical; seed thick, deeply and unequally ribbed. 
In ponds and rivers, Nebraska, Texas, Arizona and California. 
Also in Mexico, Europe, Asia, Africa and Australia. June-July. 
Oro 
ae WOLFFIA Horkel; Schleid. Linnaea, 13: 389. 1839. 
Thallus small, globose, ovoid-oblong, subcylindric or irregular, rootless, nerveless and 
leafless. The vegetative growth is from a cleft near one end of the plant, the branch being 
mostly sessile and soon detached. The ovary contains one orthotropous ovule. Fruit spher- 
ical or short ovate, smooth. Endosperm ina single layer, [Name in honor of Nath. Matth. 
y. Wolff, 1724-1784, Polish physician and naturalist. ] 
A genus of 12 species, mostly in tropical and subtropical regions. The following and two 
Mexican species, W. /ingulata and W. gladiata, comprise all the known North American forms. 
1. Wolffia Columbiana Karst. Columbia 
Wolffia. (Fig. 890. ) 
Wolffia Columbiana Karst. Bot. Unters. 1: 103. 1865-67. » 
Thallus spherical or subellipsoidal 14’/-74’’ long, with 
a limited number of stomata (1 to 6), loosely cellular and 
clear green throughout, not dotted nor gibbous. 
Floating as minute alga-like grains just beneath the sur- 
face of the water in stagnant ponds, pools and shallow lakes, 
Ontario to Connecticut and New Jersey, west to Minnesota 
and Missouri, south to,Louisiana, Alsoin Mexico and South 
America. June-July. 
2. Wolffia Brasiliénsis Wedd. Brazil 
Wolffa. (Fig. 89r.) 
Wolffia Brasiliensis Wedd. Ann. Sci. Nat. (III.)12:170. 1849. 
Thallus oblong, smaller than the last, 4’/-'4’’ long, 
flattish, densely cellular, with numerous stomata and 
dark green above, gibbous, more loosely cellular, with 
fewer stomata and paler beneath; brown-dotted through- 
out with minute pigment cells. 
Floating on the surface of stagnant waters, Ontario, Con- 
necticut to New Jersey, west to Minnesota and Missouri, 
south to Louisiana. Also in Brazil. June-July. 
Family 11. MAYACACEAE Walp. Ann. 3: 662. 1853. 
MAYACA FAMILY. 
Slender branching aquatic moss-like herbs, with linear sessile r-nerved entire 
soft leaves, notched at the apex. Flowers solitary, peduncled, white, perfect, 
and regular, the peduncles bracted at the base. Perianth persistent, consisting 
of 3 lanceolate green herbaceous sepals and 3 obovate white spreading petals. 
Stamens 3, hypogynous, alternate with the petals; filaments filiform; anthers 
24 
