6o UNIVERSITY OF MISSOURI STUDIES [203 



ices. Houstonia angustifolia, and the cliff-brakes, Pellaea atro- 

 piirpurea, P. dealbata (very rare), and the Hp-fern, Cheilanthes 

 lanuginosa, grow in shallow crannies, but only Pellaea atro- 

 purpurea is at all common. The rocks themselves are more or 

 less crusted with various lichens. 



4. Rupestres summae. The cliff-summit vegetation 

 occupies a narrow band at the verge of the cliffs. It consists 

 of a row of shrubs and dwarfed trees interspersed with the 

 cliff golden-rods, asters and other herbs. Back of this line 

 and somewhat confluent with it is a thin ribbon of prairie vege- 

 tation, or if the soil be very dry and sterile, a barrens' flora. 

 If the soil of the summits is very thin, there occur patches of 

 slight rosulate plants, such as Androsace, Draba and Alsine. 



a. Summae juniperoides. The red cedar association 

 of summit trees, shrubs, and herbs is found at the verge of 

 cliffs. There is a surprising variety of trees and shrubs. All 

 the oaks except the palustrous species are present, but most 

 characteristic are Quercus acuminata, Q. obtusiloba, and Q. 

 Marylandica. Other trees are Acer saccharinum, Fraxinus quad- 

 rangulata, Carya alba, C. amara, and Diospyros Virginiana. 

 Typical shrubs, or small trees, are Juniperus Virginiana, Ame- 

 lanchier Canadensis, Celtis occidentalis, Ostrya Virginica, Cor- 

 nus asperifolia, C. Drummondii, Viburnum rufotomentosum, 

 and Morus rubra. Vaccinium vacillans occurs also on a high 

 cliff-summit south of Grindstone creek. Solidago Drummondii, 

 S.radula, S.speciosa, Aster anomalus, A.turbinellus, A.oblongi- 

 f alius are characteristic herbs. Along the cliffs of the Missouri 

 Mentzelia oligosperma is abundant. 



b. Summae androsacoides. The rock-primrose asso- 

 ciation consists of diminutive rosettes, which occupy flat sum- 

 mits with an exceedingly shallow soil. Androsace occidentalis. 



