Plant annual, hirsutulous below, glabrous above, to about 3 dm. high; stem erect, slender, cymosely and effusively much-branched, angular, usually scabrous on the angles.
Campanula exigua is also vegetatively similar to Campanula reverchonii an annual that is endemic to granitic outcrops in central Texas (Shetler, 1963); however, its capsule and seed morphology differ strikingly from those of Campanula reverchonii. Again, convergence rather than close relationship is suggested. 2)
Leaves sparingly dentate, to about 15mm. long and 5mm. wide, the radical ones spatulate, the cauline leaves lanceolate, those of the upper branches almost filiform and entire.
May – july. Flowers and fruits erect on almost capillary elongate peduncles. Calyx lobes narrowly triangular to linear-subulate, glabrous, 2-4,5mm. long, blunt-pointed with the tips slightly thickened; corolla light-blue, narrowly funnelform, 9-13mm. long, the tube 4-7mm. long, the ovate-lanceolate lobes shorter than the tube and 6-8mm. wide. Filaments 1.3-1.5mm. long, the proximal third dilated and ciliate; anthers 1.6-2.3mm. long.
Most European Campanula pollen studied to date is 3- to 4-porate (Dunbar, 1975). Most North American campanulas (and other North American genera in Campanuloideae) have pollen that is 5- to 6-porate, the pores equatorial )Chapman, 1966; Chuang, pers. comm., 1978; Morin, unpubl. data). Only Campanula americana L., Campanula exgua, and Campanula griffinii are known to have pantoporate pollen, although pores of Campanula reverchonii are not strictly equatorial. Small (1903) placed Campanula americana in the monotypic genus Campanulastrum, and Gadella (1964) supported this decision. There is little morphological similarity between Campanula americana (which is a tall, broadleaved annual or biennial with spicate inflorescences and rotate corollas) and the annual California campanulas. Therefore it seems likely that pantoporate pollen evolved independently in these two groups.3)
Flowers and fruits erect on almost capillary elongate peduncles. Capsule ellipsoid or obovoid, 3.5-6mm. long, glabrous, crowned with the erect calyx lobes, opening above the middle.
On granite rocks and soils, endemic.
In the Llano region on the Edwards Plateau.