But far more important than any of the above phases are the extremes in temperature and more particularly of cold. A plant can not be grown profitably where the temperature, even occasionally, falls below the point where winter-killing results to tree or bud, or where the blossom is injured by frost. Extremes of heat are disastrous usually only when long continued. For each fruit, too, there must be a total amount of heat available to carry it from the setting of the fruit to maturity, in which respect varieties of any fruit may differ materially. Of the injurious effects and of the necessary amounts of heat, however, we know but little. These general considerations of temperature lead us to their application to the plum and especially to a discussion of the most important of the several factors—hardiness. Hardiness to cold, a matter of prime importance with all fruits, is especially so with the plum because of the many different species, each with its own capacity for withstanding cold. As the different species are taken from their natural habitat to other regions, there to become acclimatized, and as new forms originate by hybridization, the matter becomes more complicated and more important. Waugh[145] has investigated the hardiness of plums and we publish a table given by him showing the hardiness of representative varieties of the species most generally cultivated. We have taken the liberty of adding a few plums not given in the original table and have also made some changes in the nomenclature of the groups. TABLE SHOWING THE HARDINESS OF REPRESENTATIVE VARIETIES OF PLUMS AT VARIOUS PLACES. h—hardy; hh—half-hardy; t—tender.