By way of introduction it seems advisable to outline briefly the procedure in making a complete analysis of a sample of seed, so that readers who may not have had actual experience in seed analysis will be in a better position to appreciate the problem involved. A sample of seed as received in the seed-testing laboratory is the end product of a number of operations which may be said to commence with the seeding down of the field from which the seed crop was taken. Both the seed used at that time and the land itself contained weed seeds. The land was probably heterogeneous as to state of tilth, fertility, drainage, exposure and other factors which have a bearing upon the resulting crop. This heterogeneity is reflected in the crop as variations in vitality and viability of the seed harvested and in distribution of the contained weed seeds. Before the seed is sold it is run over cleaning machinery which brings about a considerable amount of mixing, but frequently two or more lots deriving from different farms or from different parts of the country and containing a different weed seed flora may have to be bulked or blended so as to produce as uniform a product as possible. Most of the large seed establishments have equipment for this purpose. The seed may then be bagged or stored in bulk in other types of container, and a sample must be drawn representative of the lot to constitute the "submitted" sample which is sent to the laboratory for testing. The method of taking this sample is given in detail on page 66 of the official Rules for Seed Testing (5; adopted by the Association of Official Seed Analysts, 1937) and, in somewhat different form, in the International Rules for Seed Testing, 1938, on page 408 (16). Without going into details, it may be stated that the method provides for representativeness by requiring a sufficient number of bags to be sampled or of probes to be made in the case of bulk lots. 'Contribution No. 592. Botany and Plant Pathology, Science Service, Department of Agriculture, Ottawa, Canada. 2 Botanist (Physiology) Seconded to Plant Products Division, Production Service, Ottawa. 505