232 FLORA. 



Family 2. LEMNACEAE Dumort 



Duckweed Family. 



Minute perennial floating aquatic plants, without leaves or with only 

 very rudimentary ones. The plant-body consists of a disk-shaped, elon- 

 gated or irregular thallus, which is loosely cellular, densely chlorophyl- 

 lous and sometimes bears one or more roots. The vegetative growth is 

 by lateral branching, the branches being but slightly connected by 

 slender stalks and soon separating. The inflorescence consists of one or 

 more naked monoecious flowers borne on a slight lateral prominence on 

 the edge or upper surface of the plant. Each flower commonly consists 

 of but a single stamen or a single flask-shaped pistil. The anther is 

 provided with two to four pollen-sacs, containing spherical minutely 

 barbellate grains. The pistil is narrowed to the funnel-shaped scar-like 

 stigmatic apex, and produces 1-6 erect or inverted ovules. The fruit is 

 a 1-6 seeded utricle. The family comprises the smallest of the flowering 

 plants and contains 4 genera and about 30 species of wide distribution. 



Thallus with one root or several. 



Roots several. I. Spirodela. 



Root solitary. 3. Lemna. 

 Thallus rootless. 



Thallus thick, globose to subcylindric. 3. Wolffia. 



Thallus thin, ligulate. 4. Wolffiella. 



1. SPIRODELA Schleid. 



Thallus disk-shaped, 7-12-nerved. The lateral branches subtended by a single 

 bipartite basilar rudimentary leaf. The thinly capped rootlets as well as the nerves 

 are provided with a single bundle of vascular tissue. Spathe sac-like. The ovary 

 produces two anatropous ovules. Fruit unknown. [Greek, in allusion to the cluster 

 of rootlets. Two species, the following, and S. oligorhiza, a native of the southern 

 hemisphere. 



1. Spirodela polyrhiza (L.) Schleid. Greater Duckweed. (I. F. f. 884.) 

 Thallus 2- 10 mm. long, thick, flat and dark green above, slightly convex and 

 purple beneath, palmately 5-15-nerved, bearing a central cluster of from 4-16 

 elongated roots. Rootcap pointed. In Stillwater, N. S. to Br. Col., S. Car., Tex., 

 northern Mex. and Nev. Widely distributed in the Old World and in tropical 

 America. 



2. LEMNA L. 



Thallus disk-shaped, usually provided with a central nerve and with or without 

 two or four lateral nerves. Each thallus produces a single root, which is devoid 

 of vascular tissue and is commonly provided with a thin blunt or pointed rootcap. 

 The ovary contains from one to six ovules. Fruit ovoid, more or less ribbed. 

 Endosperm in one or three layers. [Greek, in allusion to the growth of these s»all 

 plants in swamps.] About 10 species, in temperate and tropical regions. 



Thalli long-stipitate. 1. L. trisulca. 



Thalli short-stipitate or sessile. 

 Spathe open. 



Thalli i-nerved or nerveless. 



Thalli thin, without papules ; rootcap strongly curved, tapering. 



2. L. cyclostasa. 

 Thalli thick with a row of papules along the nerve ; rootcap little curved, 

 cylindric. 3. L. minima. 



Thalli 3-nerved; rootcap cylindric. 4. L. perpusilla. 



Spathe sac-like. 



Thalli green or purplish beneath ; fruit not winged. 5. L. minor. 



Thalli pale beneath, usually strongly gibbous ; fruit winged. 6. L. gibba. 



I. Lemna trisulca L. Ivy-leaved Duckweed. (T. F. f. 885.) Thallus 

 lanceolate, submerged and devoid of stomata in the primary aquatic form, ovate to 

 oblong. lanceolate, 5-15 mm. long, floating and provided with stomata in the later 

 flowering stage. The later and more common form is narrowed at the base to a 



