ZANTHOXYLUM 131 



modified stipules. Twigs moderate, rounded; pith continuous, 

 creamy-white. Buds moderate, superposed, sessile, globose, 

 woolly. Leaf-scars alternate, broadly triangular or 3-lobed; bun- 

 dle-traces 3; stipule-scars none. 



1. Z^ americanum Mill. Toothache -Tree. A shrub or small 

 tree, up to 8 m. high; nodal prickles widened at the base; leaf- 

 scars with a conspicuous articular membrane; buds red-rusty. 

 Rich woods, Quebec to North Dakota, south to Georgia and Okla- 

 homa (Fig. 173). 



PTELEA L. (Rutaceae) 



Deciduous shrubs or small trees. Twigs moderate, warty- 

 dotted; pith large, continuous, white. Buds moderate, superposed, 

 silvery-silky, low-conical, sessile, breaking through the leaf- 

 scars; terminal bud lacking. Leaf-scars alternate, relatively 

 large, U-shaped when torn by the buds; bundle-traces 3; stipule- 

 scars none. 



1. P. trifoliata L. Hoptree. A shrub or small tree, up to 

 8 m. high, without prickles; twigs buff, glabrous (or pubescent 

 in the var. mollis T. and G. ); fruit a yellowish suborbicular samara 

 with broad veiny wings, 1. 8-2. 5 cm. in diameter, persistent in 

 winter. Thickets, Quebec to Ontario and Nebraska, south to 

 Florida, Texas, Arizona, and Mexico (Fig. 174). 



AILANTHUS Desf. (Simaroubaceae) 



Smooth-barked loosely-branched deciduous trees. Twigs 

 very thick, somewhat 3-sided, with prominent lenticels; pith 

 large, roundish. Buds solitary, sessile, relatively small; ter- 

 minal bud lacking. Leaf-scars alternate, shield- shaped, large; 

 bundle-traces 9 or more; stipule-scars none. 



1. _A. altissima (Mill.) Swingle .Tree of Heaven. (A, glan- 

 dulosa Desf. ). A tree to 20 m. high, having relatively smooth 

 bark with pale stripes; twigs puberulent. Introduced from Asia, 

 widely naturalized from cultivation, often weed-like and competing 

 with native trees (Fig. 175). 



RHUS L. (Anacardiaceae) 



Deciduous shrubs or small trees, or climbing by aerial roots, 

 with milky, sometimes poisonous sap. Twigs round or obscurely 

 3-sided, slender to quite thick; pith large. Buds moderate or small, 

 solitary, sessile, round-ovoid; terminal bud often lacking. Leaf- 



