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The xeromorphic Mexican spiraeas (S. hartwegiana and S. northcraftii) are shown by cladistic and phenetic analysis to be more closely allied to Petrophytum, Kelseya, and Luetkea than to the more mesophytic Spiraea. Within Spiraea the Mexican spiraeas appear more closely related to the Old World subgenus Metaspiraea. The xeromorphic Mexican spiraeas, as well as Petrophytum, Kelseya and Luetkea, are considered to be New World derivitives of Old World Spiraea subgenus Metaspiraea that have become part of the Madro-Tertiary Geoflora, with each genus achieving some level of xeromorphic adaptation. The Mexican spiraeas are thus considered generically distinct and are placed in a new genus Xerospiraea containing one species X. hartwegiana. The more northern Spiraea northcraflii is placed in synonomy under X. hartwegiana.

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© 1985 James Henrickson

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