C. Raunkiaer, who on January 1, 1912, succeeded E. Warming as Professor of Botany in the University and Director of the Botanic Garden and Museum at Copenhagen, is one of that distinguished Danish school which has done so much to promote the study of the vegetative organs of plants in relation to their environment. His naine is primarily associated with a system of so-called "biological types" or "life-forms" and its application in phytogeography. The idea underlying this interesting and valuable system was first expressed at a meeting of the Danish Botanical Society in December 1903, and a short summary was published in 1904 (1). In a more elaborate formi it appeared in French in 1905 (2), and in a still more extended form in 1907 (3) and 1908 (4)-this last paper was translated into German by Tobler and so became more widely known than in the less accessible Danish periodicals. The following account will, it is hoped, convey to English readers a general impression of Raunkiaer's system, besides indicating the evolution of the concept on which it is based. In the 1905 paper (2) Raunkiaer reviews the work of Humboldt, Grisebach, Schouw and Engler as representing what inay be termed the floristic aspect of phytogeography, and that of Warming' on "growth-forms," and points out that something is still lacking to express the correlation between vegetation and climate. He proposes to circumscribe the domain of geographical botany as "that geographical science which endeavours to characterise the earth by its climate in so far as this is manifested by the adaptation of plants to the various seasons." As expressed later (3), he aims at finding a method of estimating the value of the climate of the various regionls of the earth by some standard which has a bearing on plant-life. The usual physical methods fail to do this, because different physical factors may have the same effect on plant growth, and the influence of various conlbinations of factors may result in essentially the same growth-form. Thus "physiological drought" may arise from widely different combinations of physical factors-cold and drought. The plant itself must be the recorder of the biological value of any climate. Any valuation used must have a uniform standard; nlothing can be gained by using cytological adaptations in one growth-form, bud protection in another, and leaf-structure in others. The factor selected by Raunkiaer is the adaptation of plants to the critical or