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



20. TRIFOLIUM. Clover 



Plants herbaceous, annual, biennial, or perennial, leafy stemmed or 

 (a few) subscapose; leaves trifoliolate, the leaflets denticulate to 

 deeply serrate; flowers in dense globose heads or short spikes, these 

 frequently subtended by an involucre, the flowers often reflexed after 

 anthesis; calyx nearly regular; corolla rose red or purple to nearly 

 white, usually persistent; pods small, usually terete, indehiscent or 

 tardily dehiscent; seeds 1 or few. 



The clovers, although palatable and nutritious to livestock and 

 beneficial for the soil, are not sufficiently abundant in Arizona to be of 

 much economic importance. Most of the native species grow in wet 

 soil along streams in the yellow pine belt. Several of them are sod 

 forming, but in the situations where they grow can have little im- 

 portance in protecting against erosion. The introduced red clover 

 (T. pratense) and alsike clover (T. hybridum) are important forage 

 plants elsewhere in the United States and in Europe. White clover 

 (T. repens), likewise an introduced species, is a useful constituent of 

 pastures and lawns in cooler climates, and the flowers yield honey of 

 the highest quality. The plants and seeds of the native species are 

 reported to have been used as food by the Arizona Indians. 



Key to the species 



1. Heads manifestly subtended by an involucre, the bracts in a whorl and usually 



more or less united (2) . 



2. Pubescence copious, at least on the peduncles and involucres; leaflets ob- 



cordate to oblanceolate; involucre saucer-shaped, the bracts united at 



least one-third of their length, their lobes entire, ovate, aristate; heads 



8 to 12 mm. in diameter, very dense; flowers not reflexed; corolla not, 



or but slightly, surpassing the calyx 1. T. microcephalum. 



2. Pubescence sparse or none (3). 



3. Heads normally not more than 10 mm. in diameter and not more than 10- 

 flowered; plant annual; stems prostrate to ascending, slender; leaflets 

 oblanceolate or obovate; involucre one-half to two-thirds as long as 

 the calyces, with usually obovate, deeply toothed lobes; lower flowers 

 becoming strongly reflexed; calyx teeth usually dark purple; corolla 6 



to 9 mm. long, much surpassing the calyx 2. T. variegatum. 



3. Heads normally more than 10 mm. in diameter and more than 10- 

 flowered (4). 

 4. Involucre cleft nearly to the base, the lobes subulate-setaceous to nar- 

 rowly lanceolate, entire; peduncles (and sometimes the petioles 

 and the upper part of the stems) often loosely and sparsely villous ; 

 leaflets oblanceolate or obovate, obtuse, truncate, or refuse, ser- 



.rulate 3. T. pinetorum. 



4. Involucre cleft usually not more than two-thirds of the distance to the 



base, the lobes oblong or deltoid, dentate, usually deeply so, the 



teeth subulate-setaceous; plant entirely glabrous (5). 



5. Heads 2 cm. in diameter or larger, corolla 12 mm. long or longer (6). 



6. Leaflets prevailingly elliptic or in the lower leaves obovate, usually 



less than 3 cm. long 4. T. fendleri. 



6. Leaflets prevailing linear-lanceolate, 3 to 4 cm. long. 



5. T. FISTULOSTTM. 



5. Heads 1 to 2 cm. in diameter; corolla 11 mm. long or shorter (7). 



7. Leaflets narrowly linear or lanceolate, often conspicuously cuspi- 



date, spinulosely serrate, often deeply so 6. T. lacerum. 



7. Leaflets oblong-lanceolate, not conspicuously cuspidate, serrulate. 



7. T. ARIZONICUM. 



1. Heads not involucrate, or the involucre greatly reduced (8). 

 8. Involucre very small but usually evident (9). 



9. Plant caulescent; stems prostrate; leaflets cuneate-obovate, glabrous or 

 glabrate; petioles, peduncles, and calyx villous; involucre minute, the 

 bracts seldom more than 1 mm. long; flowers strongly reflexed. 



8. T. amabile. 



