If one examines a random collection of pollen grains, such as might be obtained from a sample of honey (1), the muddy bottom of a pond (10, 14), or from a sticky slide exposed to the winds (23), the forms encountered are surprisingly various, as various, in fact, as the plants which produced them. There may be the one-furrowed form of the magnolia, the smooth single-pored globular grain of grass, or the three-furrowed grains of pea, rose or buckwheat; there may be the minute grains of forget-me-not, so small that they are apt to be entirely overlooked, or the enormous grains of hollyhock and four-o'clocks of several thousand times the bulk; and always there will be the large grains, with their two bladdery wings, of the pines. Some, like those of the grasses, will be smooth, while others, like those of the composites, will be bristling with spines or covered with a reticulum of vertical ridges marking the surface into a geometrical pattern. At first sight, these different forms appear to be entirely unrelated, yet they have all been derived from each other by evolutionary processes quite comparable with those whereby the plants which produced them were derived. But evolution alone does not give pollen grains their varied forms. In considering pollen grains it is necessary always to bear in mind their rather peculiar mode of formation in tetrads. Nowhere else than among pollen grains are the words of Nageli (12) truer, when he said: "A correct understanding of a thing can be gained only by a knowledge of its beginning as well as its ending." A correct understanding of pollen grains can be gained only by considering them against a background of their generation. The fact that they originate in tetrads is almost invariably the primary moulding force which gives them their completed form. Pollen grains are formed from a pollen mother-cell in a way which is almost unique in cell formation. The nucleus nearly always goes through two divisions in rapid succession, followed by two successive or simultaneous divisions of the cell, forming four