Lyaruu, H.V.M.2018-07-022023-11-082018-07-022023-11-081991-06http://etd.aau.edu.et/handle/123456789/5292A systematic study of P/ectocephalus varians (A.Rich.) C.Jeffrey ex Cuf. was conducted with an objective of obtaining data that will contribute to its relation with other members of the genus and to the related genus Centaurea L. Special emphasis was given to characters of generic value. Representative specimens of the following taxa from Latin America were studied: Plectocepha/us amerlcanus Nuttal, P. chllensis Hook & Arnott, Centaurea f/occosa Hook & Arnott, C. bulbosa Hook & Arnott, C. atacamensls (Reiche) Johnston, C. tweediei Hook & Arnott, C. rothrockii Greenm. · · and C. cachlnalens/s Phil. Although widely distributed in Ethiopia, the different populations of P. var/ans are found to constitute a single taxonomic species hence authenticating the previously established synonymy by Jeffrey (1968). P. var/ans is also found to be strikingly different from any known taxon of Centaurea in Ethiopia.Based on data from morphology, anatomy and palynology, two ecologically differentiated populations of P. var/ans are recognized: grassland and forest populations. The· grassland populations differ from forest populations in the degree of the presence of indumentum. They 'tend to have a higher degree of hairiness compared to 'forest populations. A high or low amount of hair reflects xeromorphic or mesic cdnditlons, respectively. Forest populations tend to have dark-green broad and large leaves with ff 1,: prominent brochidodromous venation, whereas grassland populations have small . shiny, light green leaves with inconspicuous venation. In the forest populations studied, stem anatomy revealed presence of af hollow pith, but in the grassland populations, the pith is filled with parenchymatolJs cells. The significance of this is associated with either xeromorphic or mesic conditions. In grassland populations, the ray florets are entirely purple, but forest populations tend to have yellow or mauve ligulate flowers. The sigrlificance of this is not clear. Pollen grains derived from grassland populations have much thicker exine layer (6.5-8.5 microns) and longer spines (3.0-4.5 microns), whereas those from forest populations have much thinner exine layer (5.5-8.0 microns) and spinules (1.5- 4.0 microns). A key to distinguish- P.- var/ans _from.the_ Latin American species of Plectocephalus Is provided. In the key Centaurea tweed/el is given as Plectocepha/us tweed/el because it sh.ares many characters with Plectocephalus. In a recent communication about this taxon Jeffrey (in lilt.) wrote that" ... Centaurea tweed/el H.& Arn .... bel ol'lgs to Chelrolophus Cass. not to Plectocephalus."enThesis