The Tropical American Genus Tachigalia Aubl. (Caesalpiniaceae)

Abstract

As in most caesalpiniaceous genera with center of distribution in the Amazon, one can divide the taxonomic history of Tachigalia into two broad periods: the first dominated by the classical works of such authors as Tulasne and Bentham, the second by the brilliant studies of Harms, Ducke, and other recent authors. Of the 22 species of Tachigalia recognized in this paper, Ducke has described six. Only one, T. alba Ducke, has been reduced to the status of variety. Throughout the history of Tachigalia most workers have readily linked it to Sclerolobium, strictly tropical American genus with center of distribution in the Amazon. Paradoxically, Bentham' segregated the two genera, placing Tachigalia in the Tribe Amherstieae and Sclerolobium as the type genus of the Sclerolobieae. He challenged Tulasne's remark as to the grande affinite between the two genera, stating that Tachigalia a quo tamen meo judicio essentialiter differt floribus obliquis et stipite ovarii calycis tubo uno adnato v. saltem supra basin affixo. In my opinion there is no basis for Bentham's segregation. The vast Tribe Amherstieae, with more than 65 genera distributed throughout the warmer climates of the world, exhibits an extraordinary meshwork of generic lines. Certainly the central position of the stipe of the ovary is not character of sufficient strength to warrant the exclusion of Sclerolobium from the tribe. In fact, the fruiting material of the two is so similar morphologically that at times it is impossible to tell them apart. Floristically, the principal differences between the two genera lie in the attachment of the stipe of the pistil, in the character of the pubescence of the perianth parts, and in the pattern of the inflorescence. One species, T. formicarum Harms, is not readily separated from Sclerolobium and may well prove strong enough link between the two genera to warrant challenging their segregation. I feel that my work stresses the structure of the flower more than has been attempted in the past. Four floral characters are of great significance in Tachigalia: (1) the shape of the receptacle-cup; (2) the shape, and, to some degree, the pubescence of the filaments of the stamens; (3) the pubescence of the petals; and (4) the general character of the stipe of the pistil. In the descriptions of many authors the great variability of the vegetative structures has been disregarded, e.g. the variation in the number of leaflets, the character of the pubescences of the leaves, the number of stipule segments, and the myrmecodomatia of the petioles. In my formal descriptions of the species of Tachigalia I refer constantly to the lax paniculate inflorescence and to the receptacle-cup as seen in hemisection. These terms may require clarification.