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 grow in warm, dry regions, being espe- 

 cially abundant in AustraUa, 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 ex\idation of several African species. 



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

 (phyllode); the piimae 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 di\dded; petals 5, more or less united at the base, or seldom 

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

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

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

 or curved, mostly 2-valved. 



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

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

 Our arborescent species are three: 



Flowers racemose, slender-pedicelled; pod nearly straight. 1. A. Wrightii. 



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



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



I. TEXAS CATS-CLAW — Acacia Wrightii Bentham 



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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, slightly hairy leaf-stalk; there are 2 or 3 pairs of pinnae 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. 



