Summary: THE need for international co-operation in recording the dates of special events in the seasonal development of a few representative species or varieties of native and established cultivated plants will be recognised when the facts are considered, that the time of such events as the opening of leaf and flower buds, the completion of growth, the ripening of fruit, the harvesting of crops, etc., are true expressions, not only of the influence represented by the three important elements of climate, temperature, precipitation and sunshine, but, in addition, that represented by all other elements in the physiographic complex of the environment affecting in any degree the seasonal development of a plant. When it is considered, further, that the correct interpretation of the geographic and economic significance of the average date of the same event, at representative places within the geographic range of the species, is an element of a true index to the bioclimatic character of each place and the immediate region represented, it will be realised that such records are of the greatest importance, especially in connexion with those of temperature and precipitation, in the study of geographic distribution of plants and animals, types of climate, cultivated plants, world agriculture, etc.