1953] 



REVISION OF MACROL OBIUM 



265 



anatomical results, cited earlier, indicate that this has been accomplished by 

 the lateral union of the adaxial pair of sepals (Figs. lb-d). Now, if the cylindric 

 hypanthium is truly advanced, then we might expect that the calyx would also 

 show the advanced sepal number of four. This is regularly true. In the two princi- 

 pal lines of development here designated as sections, the calyx has developed 

 somewhat differently. There is a tendency in the species groups of section Vou- 

 apa for those species considered to be more advanced in the total of their char- 

 acteristics to have the adaxial pair of sepals more or less reduced and often much 

 different from the others in shape. In section Stenosolen, on the other hand, the 

 sepals are about equal in size and essentially uniform in shape. 



One of the basic differences separating the two sections of Macrolobium is the 

 failure of the bracteoles to open completely on the adaxial side of the flower in 

 section Stenosolen (Figs, le, f). What selective advantage such a modification 

 could possibly possess is difficult to imagine, but it may be considered as a spe- 

 cialization, indicative of a derivation from the situation in the other section in 

 which the bracteoles open completely. 



The two sections are also easily separable by the presence or absence of a 

 claw to the single petal. It appears possible that the clawed petal of section Vou- 

 apa is the more advanced form, having originated by elongation of the basal por- 

 tion of the blade (Fig. la). On this basis, then, the section Stenosolen, more 

 highly evolved in respect to floral characters, possesses the more primitive petal 

 form. There is possible, however, an alternative hypothesis, that its subsessile 

 or sessile petal may have evolved by the progressive abbreviation of the claw 

 (Fig. le). If the latter could be demonstrated, the species of this section might 

 be looked upon as the most advanced in all their floral characters. 



In regard to developmental trends in the vegetative system, there is rather 

 clearly a progressive reduction in the number of pairs of leaflets per leaf, which 

 trend is more or less correlated with advancement in the flower. The more primi- 

 tive species of both sections have multijugate leaves, but each of the lines within 

 the sections is culminated by unijugate species. 



The diagram of relationships (Fig. 2) is a graphic representation of the fore- 

 going conclusions; the sole intent here is to indicate specific interrelationships. 

 That is, a line in the diagram from one species to another does not necessarily 

 imply that the writer believes the one species has given rise to the other. 



Literature Cited 



Amshoff, G. J. H. 1948. Caesalpiniaceae [of Guiana]. In: Maguire, E>. et al. Plant explora- 

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 Bull. Torrey Club 75:387-392. 



Aublet, J. 1775. Histoire des plantes de la guiane francoise 1:2 5-30; 2 :pl. 7-9. 



Bentham, G. 1870. Caesalpinioideae. In: Martius, Flora brasiliensis 15(2):2 17-224. 



Britton, N. L. & Killip, E. P. 1936. Mimosaceae and Caesalpinaceae of Colombia. Ann. 

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Ducke, A. 1941. Revision of the Macrolobium species of the Amazonian Hylaea. Trop, 



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 Kuntze, O. 1891. Revisio generum plantarum 1:213. 



Lanjouw, J. & Stafleu, F. A. 1952. Regnum regetabile: Index herbariorum, part 1. Int. 



Bur. Pi. Tax. and Nomencl., Utrecht, Netherlands. 

 Pittier, H. 1941. Especies venezolanas de Macrolobium. Bol. Soc. Venez. Ci. Nat. 



7:138-145. 



Taubert, P. 1891. Zur nomenclatur einiger genera und species der Leguminosen. Bot. 

 Centralbl. 47:393-394. 



