A Revision Of The Genus Plectocephalus In Ethiopia

A systematic study of P/ectocephalus varians (A.Rich.) C.Jeffrey ex Cuf. wasrnconducted with an objective of obtaining data that will contribute to its relation withrnother members of the genus and to the related genus Centaurea L. Special emphasisrnwas given to characters of generic value. Representative specimens of the followingrntaxa from Latin America were studied: Plectocepha/us amerlcanus Nuttal, P. chllensisrnHook & Arnott, Centaurea f/occosa Hook & Arnott, C. bulbosa Hook & Arnott, C.rnatacamensls (Reiche) Johnston, C. tweediei Hook & Arnott, C. rothrockii Greenm.rn· · and C. cachlnalens/s Phil. Although widely distributed in Ethiopia, the differentrnpopulations of P. var/ans are found to constitute a single taxonomic species hencernauthenticating the previously established synonymy by Jeffrey (1968).rnP. var/ans is also found to be strikingly different from any known taxon ofrnCentaurea in Ethiopia.Based on data from morphology, anatomy and palynology,rntwo ecologically differentiated populations of P. var/ans are recognized: grassland andrnforest populations. The· grassland populations differ from forest populations in therndegree of the presence of indumentum. They 'tend to have a higher degree ofrnhairiness compared to 'forest populations. A high or low amount of hair reflectsrnxeromorphic or mesic cdnditlons, respectively.rnForest populations tend to have dark-green broad and large leaves withrnff 1,:rnprominent brochidodromous venation, whereas grassland populations have smallrn. shiny, light green leaves with inconspicuous venation. In the forest populationsrnstudied, stem anatomy revealed presence of af hollow pith, but in the grasslandrnpopulations, the pith is filled with parenchymatolJs cells. The significance of this isrnassociated with either xeromorphic or mesic conditions. In grassland populations, thernray florets are entirely purple, but forest populations tend to have yellow or mauvernligulate flowers. The sigrlificance of this is not clear. Pollen grains derived from grassland populations have much thicker exinernlayer (6.5-8.5 microns) and longer spines (3.0-4.5 microns), whereas those fromrnforest populations have much thinner exine layer (5.5-8.0 microns) and spinules (1.5-rn4.0 microns).rnA key to distinguish- P.- var/ans _from.the_ Latin American species ofrnPlectocephalus Is provided. In the key Centaurea tweed/el is given as Plectocepha/usrntweed/el because it sh.ares many characters with Plectocephalus. In a recentrncommunication about this taxon Jeffrey (in lilt.) wrote that" ... Centaurea tweed/el H.&rnArn .... belrnrnol'lgs to Chelrolophus Cass. not to Plectocephalus."

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