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Turesson (14, 15, 16) demonstrated that species occurring in a wide range of climates are composed of genetically distinct races, and developed the concept that such races are products of natural selection. This concept is fundamental to an understanding oi mechanisms underlying natural selection and evolution of higher plants. Extensive later experiments bv other wvorkers on the response and survival of climatic races transplanted in contrasting environments have established Turesson's concept as a general principle. Another advance in the study of climatic races is the clarificatioin of major aspects of their comlplex genetic structure (3, 4, 5, 6). In addition to the infornmation being provid(led by field, transplant, ancI genetic investigations, further elucidlation of the nmechanisms of natural selection requires knowledge of the physiology of climatic races. Working principles for a physiological approaclh to the problemii were reviewved by Hiesey, MIilner, and( Nobs (8). Of the many species subjected to tralnsplanit and(l genetic study, members of the Minmulus cardinalislewisii species complex are exceptionally well suited for physiological study. These plants range in habitat froml Mlexico to southern Alaska and from near sea level to alpine conditions. They show a wi(le diversity in ability to survive in contrasting climates. Vickery (17) demonstrated that AM. cardinialis Dougl. and Al. lezeisii Pursh., classified as clistinct species, are in reality memnbers of a single genetically conmpatible species complex. Nobs anid Hiesey (12, 13) confirmiied by crossing and transplant experiniiients that this species conmplex contains a widle diversity of readlil) recognized climiatic races. The plants, easily propagated vegetatively as clones, have leaves well suite(d for physiological measurements. The present paper is linmited to the effect of light intensity and temperature on the photosynthetic rates of 6 climatic races of Al. cardinalis native to 6 different habitats. Our first objective is to discover whether the rates of the races differ over a wide range of external con(litions. The kinetic and( physiological aspects of the photosynthetic process in iNlimulus receive only incidental attentioii.

Photosynthesis in Climatic Races of Mimulus. I. Effect of Light Intensity and Temperature on Rate