A new nothospecies of moonwort (Botrychium subg. Botrychium) is reported from Waterton Lakes National Park, Alta., Canada, where it is associated with its putative parents, B. paradoxum W. Wagner and B. hesperium (Maxon and Clausen) Wagner and Lellinger. A population of 29 individuals of the hybrid was found. The plant is here named and described as B. × watertonense W. Wagner nothosp. nov. Morphologically it is remarkable for displaying leaf characteristics intermediate between B. paradoxum, which has two sporophores but no sterile lamina, and the more typical B. hesperium with a single sporophore plus a sterile lamina. The "sterile lamina" of B. ×watertonense is much contracted, has simple venation, and produces sporangia along the margins from base to apex. Flavonoid chemical constituents of the nothospecies compared with its putative parents corroborate the conclusion of its hybrid origin based on occurrence and morphology. The presence of so many individuals at the type locality of a presumably sterile nothospecies is surprising in view of the fact that vegetative propagation is unknown in this genus. In spite of their obvious abortion, there may be some mode of reproduction by spores.
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