Myrceugenia glaucescens (Cambess.) D.Legrand & Kausel

• Authority

  Landrum, Leslie R. 1981. A monograph of the genus Myrceugenia (Myrtaceae). Fl. Neotrop. Monogr. 29: 1-137. (Published by NYBG Press)

• Family

  Myrtaceae

• Scientific Name

  Myrceugenia glaucescens (Cambess.) D.Legrand & Kausel

• Synonyms

  Eugenia glaucescens Cambess.

• Description

  Species Description - Shrub or small tree 2-10 m high; hairs reddish-brown to whitish, appressed, more or less symmetrically dibrachiate; twigs glabrous to sparsely pubescent when young, soon glabrescent; leaves narrowly elliptic, obovate or oblanceolate, (2-)2.5-8 cm long, 0.5-3 cm wide, (1.5-)1.8-5(-8) times as long as wide, glabrous or with a few scattered hairs; apex acuminate, acute to rounded; base acuminate, cuneate or acute; petiole channeled, glabrous to moderately pubescent, 2-8 mm long, 0.5-1 mm thick; midvein impressed for entire length, only proximally or not at all above, prominent below; lateral veins indistinct or up to ca. 20 pairs barely visible; marginal veins equalling laterals in prominence; blades grey-green to reddish-brown above, lighter grey-green, yellow-green to light yellow below, coriaceous to submembranous, the upper surface dull to slightly lustrous; peduncles uniflorous, flattened, 1-3 cm long, 0.5-1 mm wide, glabrous to moderately pubescent, solitary or in pairs in the leaf axils; bracteoles ovate to lanceolate, (0.9-)1.3-2(-2.4) mm long, (0.5-)0.8-1.6(-2) mm wide, 1.2-2 times as long as wide, glabrous to very sparsely pubescent within and without, subcoriaceous, clasping the hypanthium; Calyx-lobes more or less hemiorbicular, 1.6-3 mm long, 1.2-3.5 mm wide, 0.8-1.4 times as long as wide, densely pubescent to glabrous within, sparsely pubescent to glabrous without, subcoriaceous to coriaceous, concave; hypanthium densely pubescent to glabrous, 1-3 mm long, obconic, the sides sometimes slightly concave; disk 2-3 mm across, glabrous to sparsely pubescent; style 6-10 mm long, glabrous to sparsely pubescent; stamens 140-250 (-270), 3-10 mm long; anthers 0.3-0.6 mm long when dry; petals suborbicular, concave, 3-5 mm in diam.; ovary 2-4-locular; ovules (4-)6-9(-13) per locule; fruit globose to slightly elongate, 5-12 mm in diam., dark reddish-brown to purple; seeds 1-5 per fruit as far as known, oblong, 4-6 mm long.

• Discussion

  Of evolutionary interest but of taxonomic annoyance is the existence of a continuum of forms that includes Myrceugenia glaucescens and the eastern population of M. ovata: M. ovata var. gracilis and M. ovata var. acutata. The ends of the continuum (excluding the western population of M. ovata) are M. ovata var. gracilis and M. glaucescens var. glaucescens which are quite different, grow sympatrically, and show no signs of hybridization. But M. ovata var. acutata and M. glaucescens var. latior seem to merge with one another, as is shown in the scatter diagram in Fig. 12. Each point represents one specimen and the numerical values are averages of a few measurements. The points can be divided almost exactly according to limits of the taxa recognized here, section A corresponding to M. ovata var. gracilis, B to M. ovata var. acutata, C to M. glaucescens var. latior and D to M. glaucescens var. glaucescens. For each group the average stamen number, locule number, and ovule number has been calculated and these are given in Table V. Clearly there are no sharp breaks in the characters analyzed here.

  There are three possible explanations for the continuous nature of the variation in this group. First, the complex could be a phylogenetic continuum, i.e., a cline that has turned back on itself so that the ends are growing together and acting as separate species (Fig. 13A). Second, parallel evolution could have given rise independently to two forms that are so alike that they are inseparable (Fig. 13B). Third, two different phylogenetic lines could have merged to form a hybrid which links them together (Fig. 13C). Both Myrceugenia glaucescens and M. ovata have apparently close relatives that are included in the diagrams illustrating these hypotheses (Fig. 13).
  If the first hypothesis is accepted then it is necessary that the continuum in eastern South America must be ancestral to one or both of the groups of related species. But the continuous nature of their variation would indicate that members of the continuum have only begun to diverge from each other, an unlikely situation for an ancestral group. Such extraordinary parallel evolution as the second hypothesis entails does not seem likely either. Because hybridization is known to occur at present between other species I suggest that the third hypothesis is the most probable, but the question has by no means been answered with certainty. Perhaps some future taxonomist with more material and additional data will study the group again.