176 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 

 1. SPIRODELA. Duckweed 



Thallus broadly obovate, 7- to 15-nerved; anthers longitudinally 

 dehiscent; ovules 2; fruit rarely seen. 



1. Spirodela polyrhiza (L.) Schleiden, Linnaea 13: 392. 1839. 



Lemna polyrhiza L., Sp. PL 970. 1753. 



Huachuca Mountains (Lemmon in 1882). Almost cosmopolitan. 



2. LEMNA. Duckweed 



Thallus orbicular to oblong or lanceolate, 1- to 5-nerved or almost 

 nerveless; anthers transversely dehiscent; ovules 1 to 6; fruit ovoid. 



Key to the species 



1. Blades long-stalked at base, remaining connected in chains, 6 to 10 mm. long, 



oblong or lanceolate, mostly submerged 1. L. trisulca. 



1. Blades sessile or nearly so, soon separating, not more than 5 mm. long, float- 

 ing (2). 

 2. Lower surface of the blades strongly gibbous and much paler than the upper 



surface; blades suborbicular or obovate, asymmetric 2. L. gibba. 



2. Lower surface of the blades flat or nearly so (3). 



3. Blades oblong-obovate or suborbicular, symmetric or nearly so, obscurely 



3-nerved; fruit approximately lenticular 3. L. minor. 



3. Blades oblong or elliptic, indistinctly 1-nerved or nerveless; fruit more or 

 less elongate (4) . 

 4. Papillae present on the blades, these symmetric, 1.5 to 3 mm. long. 



4. L. MINIMA. 

 4. Papillae absent, the blades asymmetric at base, 2.5 to 4 mm. long. 



5. L. VALDIVIANA. 



1. Lemna trisulca L., Sp. PL 970. 1753. 



Willow Spring, southern Apache County, 7,200 feet (Palmer 531). 

 Nova Scotia to New Jersey, Texas, and west to the Pacific coast; in 

 all continents except South America. 



2. Lemna gibba L., Sp. PL 970. 1753. 



Pinal, Maricopa, Cochise, and Pima Counties. Nebraska to Texas, 

 Arizona, and California; almost cosmopolitan. 



3. Lemna minor L., Sp. PL 970. 1753. 



Apache, Pinal, Maricopa, and Pima Counties. In the greater part 

 of North America; almost cosmopolitan. 



4. Lemna minima Phil, ex Hegelm., Lemn. 138. 1868. 



Ruby to Nogales, Santa Cruz County, 4,200 feet (Kearney and 

 Peebles 14486). Southwestern United States to South America. 



5. Lemna valdiviana Phil., Linnaea 23: 239. 1864. 



Oak Creek Canyon, Coconino or Yavapai County (Goodding 13). 

 Widely distributed in North and South America. 



13. BROMELIACEAE. Pineapple family 



1. TILLANDSIA 



Plant epiphytic (growing on the branches of trees but not para- 

 sitic); leaves 2-ranked, crowded, awl-shaped from an enlarged base, 

 scurfy-canescent ; inflorescence spicate; flowers perfect, regular, the 



