522 The Acacias 



VI. THE ACACIAS 



GENUS ACACIA [TOURNEFORT] ADANSON 



CACIA comprises about 450 species of trees, shrubs, and a few herbs. 

 They are mostly armed and giow in warm, dr)' regions, being espe- 

 cially abundant in AustraHa, where some 300 species occur and where 

 they are among the most valuable trees. There are no native living 

 forms in Europe, although numerous fossil ones are known from the lower Eocene 

 formation of that continent. In addition to the arborescent species in our area, 

 there are about 10 shrubby species along our southern border. The well-known 

 Gum arable is a natural gummy exudation of several African species. 



The leaves are usually evenly bipinnate, sometimes reduced to a dilated petiole 

 (phyllode); the pinnse are numerous, often with many leaflets; the stipules are small, 

 deciduous or spinescent. The flowers are in globose heads or cylindric racemes 

 or spikes, variously clustered, terminal or axillary, more or less bracteate, usually 

 perfect, sometimes polygamous, small, yellow or white; the calyx is bell-shaped, 

 5-toothed, lobed or divided; petals 5, more or less united at the base, or seldom 

 separate, rarely wanting; stamens 50 or more, exserted, free or but Httle united, 

 filaments thread-hke; anthers small; ovary sessile or stalked, 2- to many-ovuled, 

 contracted into the long style. The fruit is oblong or hnear, usually flat, straight 

 or cun'ed, mostly 2-valved. 



The name is from the Greek, in reference to the spiny branches of many of 

 these plants. The African Acacia nilotica Dehle is the type species. 



Our arborescent species are three: 



Flowers racemose, slender-pedicelled; pod nearly straight. i. A. Wrightii. 



Flowers spicate, sessile; pod much curled and contorted. ?. A. Greggii. 



Flowers capitate; pod straight, but slightly compressed. 3. A. subtortuosa. 



I. TEXAS CATS-CLAW -Acacia Wrightii Bentham 



A small tree or shrub, with dense foliage and short spines, occurring in grav- 

 elly soil, from western Texas southward into Mexico, often becoming 9 meters 

 high, with a trunk diameter of 3 dm. 



The branches are spreading, and form an irregular tree; the bark is about 

 3 mm. thick, furrowed into ridges, which are broken into dark gray-brown scales; 

 the twigs are somewhat angular, smooth, yellowish or reddish brown, soon be- 

 coming pale gray, and armed with stout recurved spines 4 to 6 mm. long. The 

 leaves are evenly bipinnate, 2 to 3.5 cm. long, including the slender, sometimes 

 glandless, sHghtly hairy leaf-stalk; there are 2 or 3 pairs of pinna? 2 to 2.5 cm. 

 long, and short stalked; the leaflets, 2 to 6 pairs, are sessile or nearly so, ob- 

 liquely oblong to obovate, 5 to 7 mm. long, rounded, blunt or short-pointed at the 

 apex, stiff, hght green and smooth above, paler with prominent venation beneath. 



