456 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
These differences are fundamental, and sufficient to characterize the 
groups. 
Pithecollobium hymenaeaefolium is distinguished from P. ligus- 
trinum by its large flowers, each with a strikingly developed stamen 
tube. P. macrostachyum, an imperfectly known species, is, accord- 
ing to description, a closely similar if not identical form. In the 
group of P. ligustrinum, with six easily distinguishable speciés or 
subspecies, the species are separated primarily by the size and nature 
of the stipules, the shape of the bractlets, the presence or absence of 
indument on the ovary, and other characters which seem to be very 
constant. 
With the well-characterized group of P. oblongum an approach is 
made to the globose-flowered species of Pithecollobium of the same 
section. This group presents two almost parallel series, differenti- 
ated by a single but apparently constant character, the length of 
the ovary stipe. In each group we have two species, the one with 
stipitate petiolar glands, the other with sessile glands. The number 
of flowers in each spike is much reduced in the last two species 
of the series, the more so in P. subglobosum, which might be placed 
among either the Spicatae or the Globosae but for their possession 
of characters showing their affinity with P. oblongum. 
KEY TO SPECIES. 
Legume short, terete, and very thick, the valves about 2 cm. broad. Petioles 
very short; calyx and corolla grayish-hairy________ _a---- 1, P. pachypus. 
Legume rather long and slender, sometimes flat, the valves narrow. 
Ovary sessile; spikes usually single in the axils; stamen tube usually exserted. 
Corolla 8.5 mm. long or more; stamen tube very long-exserted. 
Stamen tube seldom over 2.5 em. long; corolla 12 to 13 mm. long; 
leaflets ovate-lanceolate, up to 13 cm. long_. 2. P. hymenaeaefolium. 
Stamen tube up to 3.8 cm. long; corolla 8.5 to 11 mm. long; leaflets ovate- 
oblong, up to 8 em. long____________________- 3. P. macrostachyum. 
Corolla 7 mm. long or less; stamen tube moderately exserted. 
Stipules spinulose or obsolete; ovary glabrous______ 4. P. spinulosum. 
Stipules spinescent, strong, rarely obsolete; ovary pubescent or hairy. 
Bractlets lance-subulate, subpersistent, often as long as the calyx. 
Corolla 6 to 7 mm. long; stamen sheath exserted about 2 mm. 
5. P. insigne. 
Corolla 5 to 6 mm. long; stamen sheath exserted about 5 mm. 
6. P. calostachys. 
Bratlets small, scalelike, much shorter than the calyx. 
Calyx 1 to 12 mm, long___-_-_______________ 7. P. macrosiphon. 
Calyx 2 to 3 mm. long. 
Stipular spines long (up to 2 cm.) ; interpetiolar stipules subulate; 
corolla minutely pubescent_____.__-_--____- 8. P. lanceolatum., 
Stipular spines short (seldom over 0.5 cm.) ; interpetiolar stipules 
acute-triangular and very short; corolla conspicuously silky- 
pubescent ___-_------------------------- 9. P. ligustrinum. 
