DUCKWEED FAMILY. 



Family 10. LEMNACEAE Dumort. Fl. Bclg. 147. 1827.' 

 DUCKWEED FAMILY. 





Minute perennial floating aquatic plants, without leave* or with only very 

 rudimentary ones. The plant body consists of a dwc-shaped elongated or lrr- 



ular thallus. which is loosely cellular, densely chloniph 

 bears one or more rootlets. The vegetative growth is by lateral branching, the 

 branches being but slightly connected by slender stalk* and soon crarating 

 In the autumn these disconnected branches fall to tin- 

 pond, but rise and again increase in si/.e in the spring. 'I 

 sists of one or more naked monoecious flowers l>oni> -ht lateral promi- 



nence on the edge or upper surface of the plant. Ivu h flower commonly con- 

 of but a single stamen or a single flask-sha^l pistil. The ..mlirr i pro- 

 vided with two to four pollen-sacs, containing spherk-.il minutcK barbaHtC 



grains. The pistil is gradually narrowed to the hum. 



apex, and produces 1-6 erect or inverted ovules. The fruit is a i -6-*eeded utricle. 



The family comprises the smallest of the flowering plants and contain* \ rrnrrm, all erf 

 are represented in North America, and about 26 spo it-s i,f \\ulr <h-ti!t". 

 Thallus prominently nerved; with a membranous spathe and a cluster of several rootlet*. owl** 



Thallus slightly nerved; with a membranous spathe and a single rootlet; nrale* 1-6* *. Yimmt 



Thallus nerveless; without spathe or rootlets; ovule I. :fk*. 



i. SPIRODELA Schleid. Linnaea, 13: 391. is 



Thallus disc-shaped, 7-i2-nerved. The lateral branch*-, subtended by ingle bipartite 

 basilar rudimentary leaf. The thinly-capped rootlets as well as the nerve* are provkUd 

 with a single bundle of vascular tissue. The ovary produces two anatropou* ovule*. Pratt 



unknown. [Greek, in allusion to the cluster of rootlets.] 



Two species, the following, and 5. oligorhiza, a nati\< 

 of southern Asia, Australia and the Fiji Islands. 



i. Spirodela polyrhiza (L. ) Schleid. 

 Greater Duckweed. (Fig. 884. ) 



Lemna polyrhiza L. Sp. PI. 970. 1753. 



Spirodela polyrhiza Schleid. Linnaea, 13: 392. 1839. 



Thallus round-obovate, 2 // -5 // long, thick, flat and 

 dark green above, slightly convex and purple beneath, 

 palmately 5-n-nerved. Each thallus bears a central 

 cluster of from 5-11 elongated rootlets. Rootcap 

 pointed. 



In rivers, ponds, pools and shallow lakes, Nova 

 to British Columbia, south to South Carolina, Texas, 

 northern Mexico and Nevada. Widely distributed in the 

 ( )ld World and in tropical America. Seldom collected in 



flower. 



2. LEMNA L. Sp. PI. 970. i; 



Thallus disc-shaped, usually provided with a central nerve and will 

 four lateral nerves. Each thallus produces a single rootlet, which i* dew* 

 sue and is commonly provided with a thin blunt or pointed 

 from one to six orthotropous amphitropous or anatropous o-. ut <m 



* .1 1 r /"** VAA!- in alltiOfkfl to la 



ribbed. Endosperm in one or three layers. [Greek, in 

 small plants in swamps.] 





South America. 



Ovule i, amphitropous or orthotropous; endosperm in three layer 

 Thalli of two kinds, ovule amphitropous. 

 Thalli all alike. 



Thallus without lateral nerves; ovule ortli 

 Thallus with lateral nerves; ovule orthotropoi 

 Thallus with lateral nerves; ovule amphitropous. 

 Ovules 2-6, anatropous; endosperm in one layer. 



*Text contributed by Mr. EDMUND P. SHELDON. 





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