Crusea calcicola Greenm.

Authority

Anderson, William R. 1972. A monograph of the genus Crusea (Rubiaceae). Mem. New York Bot. Gard. 22: 1-128.

Family

Rubiaceae

Scientific Name

Crusea calcicola Greenm.

Description

Species Description - Erect annuals 6-50 cm tall (to 100 cm in greenhouse conditions); stems quadrangular with ribs on the angles, usuaHy hispidulous, the hairs 0.1-0.5 mm long, unicellular, restricted to or concentrated on the angles; leaf blade 14-45 mm long, (1.5-) 2.5-17 mm wide, 2-10 times as long as wide, linear or narrowly elliptic to broadly elliptic or ovate, acute at the apex, gradually narrowed at the base, sessile or with a winged petiole up to 4 mm long, scabrous on both surfaces; stipular sheath 1-4(-5) mm long, 1.5-4.5(-6) mm wide at the top, bearing 4-9 stipular setae and 0-2 small sessile lateral coHeters; setae evenly spaced or, usually, some of them clustered in a central group with fused bases, glabrous or, usually, hispidulous basally or their whole length with hairs up to 0.3 mm long; longest seta 1-4 mm long; flowers borne in bracteate heads, with up to 30 flowers per head, up to a third of the corollas open at one time; bracts usually 8; pales numerous, 2-4 mm long, linear, vascularized, bearing many long, fine hairs; hypanthium 0.7-1.2 mm long, glabrous, with many small raphide-cells ; calyx usually about 1/2 as long as the corolla tube, to equalling it, calyx tube 0.4-0.9(-1.3) mm long, the lobes 1-2.3(-2.7) mm long and 0.4-1.2 mm wide, narrowly to broadly triangular, rotate in fruit; sinus colleters 1; abaxial surface of the calyx glabrous, adaxial surface glabrous or sparsely strigillose in the throat, margins of the lobes basally ciliate with long fine hairs diminishing in abundance and length to none or few and very short near the apex; calyx deciduous in fruit; corolla pure white, the tube 2.3-3.7 mm long, rather narrowly tubular, gradually flared in the distal 1/2 to a throat 0.6-0.8 mm across, externally glabrous or papillose, internally fiHed in the distal 1/2-2/3 with long, straight-sided hairs; lobes 1-1.7 mm long, 0.5-0.9 mm wide, triangular or ovate, erect or spreading, abaxially bearing few to many blunt hairs on the distal half or near the apex, adaxially glabrous, with 3 main longitudinal veins anastomosing only at the apex and often a shorter longitudinal vein at each side; filaments 0.7-1.4 mm long, shorter than to as long as the corolla lobes, glabrous, neither retracted nor reflexed after anther dehiscence; anthers white, 0.6-1 mm long; pollen 47-53 microns in diameter at the equator, somewhat oblate, the colpi 11-14(-15) and short, 19-24% of the pole-to-pole arc; placenta apically bilobed, bent over the top of the ovule and under its base; ovule with a prominent apical cleft; style 2.5-4.8 mm long, glabrous, the stigmatic lobes 0.2-0.5 mm long; cleistogamous flowers occasional on plants collected in the field, very abundant on greenhouse plants, mixed in heads of chasmogamous flowers, especially the later-formed heads, consisting of normal buds with arrested elongation of the corolla and internal maturation of the anthers and stigmas; carpophore bifid, with the base 0.3-0.8 mm long and the lobes 0.6-1.3 mm long; cocci light brown to reddish-brown, 1.3-2.6 mm long, 1-1.6 mm wide, 1.1-1.7 times as long as wide, usually turbinate with a broad flat top, occasionally nearly cylindrical; coccus waH membranous and tightly adherent to the seed, glabrous, with many raphide-cells abaxially; seed with a shallow to moderately deep apical-dorsal invagination of the testa; embryo in longitudinal section moderately to strongly incurved following the curvature of the seed and the apical-dorsal invagination; cotyledons in cross section wide, slightly bent medianly by the apical-dorsal invagination and with their edges incurved; chromosome number: n=(27?) 28. Flowering and fruiting dates. Flowers in August, September, and probably October; collected with mature fruits from October through November or early December.

Discussion

Type: Pringle 4869 (GH! holotype; BM! BR! F! MICH! MO! NY ! US! isotypes).

Figs. 10, 28.

Distinguishing characteristics. Stems quadrangular, the hairs very short and concentrated on the angles; stipular sheath about as wide as long, bearing setae about as long as the sheath or shorter, the setae usually at least basally hispidulous and somewhat clustered; heads with 8 bracts and rather few flowers; pales bearing long hairs; calyx rotate in fruit; coroHa white, the tube rather narrow and gradually flared, the lobes with at least a few blunt abaxial hairs; filaments about as long as the corolla lobes, not retracted after anther dehiscence; pollen somewhat oblate, with rather short colpi; placenta apically bilobed and bent over the top of the ovule; cocci usually turbinate, the coccus wall membranous and tightly adherent to the seed, containing raphides; seed with a shallow to moderately deep apical-dorsal invagination; cotyledons moderately wide, with their edges incurved.

Reproductive biology. Crusea calcicola has protandrous flowers, but residual pollen is rather close to the maturing stigmas because the stamens are not retracted after anther dehiscence. Cleistogamous corollas are present in the collections seen, although much less abundant than in the plants grown in the greenhouse. These facts suggest a variable amount of inbreeding, with a balance probably more toward outbreeding than C. diversifolia, with its many specialized cleistogamous flowers, but more toward inbreeding than C. setosa, which has very limited cleistogamy and strong retraction of the stamens.

Discussion. There are two problems involved in understanding this species, neither of which I can satisfactorily resolve with the information available. The first problem is that there is great variation in leaf shape, with the extremes being linear versus broadly elliptical or ovate. Plants grown in the greenhouse retained these differences, and m y first inclination was to describe the narrow-leaved plants as a new species or variety. I eventually decided that this would be premature because (1) the two types are alike in most other respects, (2) they seem to grow in the same areas, if not actually in mixed populations, and (3) very few collections are known, but even among them plants with leaves of intermediate width occur. Until this species is more collected and better understood, it seems best to treat it as one taxon.

The other problem concerns the relationship of this species to other species in the genus. Crusea calcicola shows a mixture of various characteristics of four other species, viz. C. diversifolia, C. setosa, C. psyllioides, and C. longifiora. Few if any of its characteristics are uniquely its own, and yet its combination of characteristics is quite distinct from all of the species cited above. This state of affairs suggests that C. calcicola may be of hybrid origin, or may at least experience periodic introgression from other species, and it is a suggestive fact that most of the known populations were growing with C. diversifolia. Introgression with C. diversifolia would be particularly helpful in explaining the linear leaves of some plants of C. calcicola. On the other hand, gregariousness is common among weedy species of Crusea, and cannot be regarded as very good evidence of hybridization. As far as I know, C. calcicola has normal meiosis, good pollen, and perfectly viable seeds. In short, although hybridization between or with other species might make C. calcicola easier to understand, I have no evidence that it actually occurs. There is an alternative interpretation of this species' morphological intermediacy which must be seriously considered. In any group of closely related plants, there are often species that seem to "recombine" characteristics of other species. This may result from a certain amount of parallel evolution, but it may also indicate a true evolutionary intermediacy and interrelationship that is highly significant. If that is the case with C. calcicola (and I am now inclined to think that it may well be), then this species assumes a position of great interest and helpfulness in interpreting the evolutionary history of the genus.

The following specimens are probably referable to C. calcicola, but they were young and quite sterile when collected, and it is possible that some or all of them belong in some other genus of the Spermacoceae; all were collected in 1968, and all are at MICH. Guerrero: limestone hill ca 9 mHes N of Iguala, 1450-1700 m, 26 Jul, Anderson & Anderson 4899; 4.1 miles W of Chilpancingo, 1600 m, 27 Jul, Anderson & Anderson 4909. Oaxaca: hill S of Matatlan, 2050 m, 14 Jul, Anderson & Anderson 4757 & 4758; 6 miles S of Miahuatlan, 1850 m, 15 Jul, Anderson & Anderson 4771; road to Guelatao de Juarez, 6.5 miles N of intersection with Mexico highway 190, 1850 m, 18 Jul, Anderson & Anderson 4816; road to Guelatao de Juarez, 9.2 miles N of intersection with Mexico highway 190, 2200 m, 18 Jul, Anderson & Anderson 4819; several miles E of Ixtlan de Juarez, 1750 m, 19 Jul, Anderson & Anderson 4853; lower slopes of Cerro San Felipe, above San Felipe del Agua, 1700-2000 m, 20 Jul, Anderson & Anderson 4860.

Distribution

Habitat. All known collections are from calcareous, hilly places with open or sparsely shrubby vegetation, at elevations of 1500 to 2100 m. Distribution. (Fig. 27) As the m a p shows, the collections of this species are too few and too scattered for m e to make an informed statement as to its distribution, except that it seems to be limited to south-central Mexico north of the Sierra Madre del Sur.

Mexico North America|