When assessing the value of any variety of a commercial plant one is concerned with a number of different features of the plant's behaviour, among others winter-hardiness. The behaviour with regard to most of these features is tested by methods which have been carefully planned and whose reliability has been proved, but the determination of winter-hardiness is still a matter of observing the reaction of the plants to the conditions prevailing at that season of the year. However, in England, particularly in the southern counties, the winters are comparatively mild and it is only occasionally that the climate provides a really good test of the plant's resistance. On the other hand it is necessary to have some knowledge of the winter-hardiness of a variety, since otherwise disastrous results may follow on a really hard winter. A method of determining the resistance of new varieties in the laboratory would provide the solution to this problem.