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Genus Isothecium in Pacfic North America Williams, Robert David

Abstract

An analysis was made of the variation in the genus Isothecium from Pacific North America. The study consisted of herbarium work and experimental studies, including coldframe cultures, transplant experiments and aseptic spore cultures. Within the designated area Isothecium was found to consist of a complex of ecotypes or forms, each distinct in growth-form and leaf morphology. Intermediates were found between the forms. The growth-forms were phenotypically plastic, and resembled each other under uniform culture conditions. Leaf morphology remained constant even under culture and is therefore a useful taxonomic character. The study is illustrated by twelve plates of photographs, three pages of line drawings and three distribution maps. A hypothesis is given to explain the disjunct distribution of Isothecium stoloniferum var. spiculiferum, and var. myurellum. As a result of an extensive study of nomenclature Isothecium spiculiferum (Mitt.) Ren. & Card. (Crum, Steere & Anderson 1964) was found to be an illegitimate name. An argument is presented to demonstrate that by a strict application of the Rules of Botanical Nomenclature (1961), Isothecium stoloniferum Brid. is the first available legitimate name. Three varieties are recognized under Isothecium stoloniferum: var. stoloniferum, var. spiculiferum and var. myurellum. Isothecium cristatum (Hampe) Robinson is recognized as a distinct species composed of a complex of ecotypes, which are not recognized here as clearly defined varieties until further experimental work is done.

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