White vervain has opposite, simple leaves on thin, rigid, green stems. The leaves look similar to those of Urtica, which is the reason for the plant being named urticifolia. The small flowers are borne in spikes; they open in summer and unusually for this normally bluish-flowered genus are white. The fruit is a dark- colored capsule with many brown and thin seeds. The entire plant except for the flowers and fruit is covered in stiff bristles.
Relation to other vervains
It might be closest to a group that might include such North American species as V. lasiostachys or V. menthifolia, and the common vervain (V. officinalis) from Europe. As these, it is diploid with 14 chromosomes altogether. The relationship of the swamp verbena (V. hastata) to these species is more enigmatic; its evolution might have involved hybridization with the White Vervain or a related species in the past.
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- Yuan, Yao-Wu & Olmstead, Richard G. (2008): A species-level phylogenetic study of the Verbena complex (Verbenaceae) indicates two independent intergeneric chloroplast transfers. Molecular Phylogenetics and Evolution 48(1): 23-33. doi:10.1016/j.ympev.2008.04.004
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