The rates of early seed development were compared in several species in the Triticeae which play a major role in human nutrition, and in several related genotypes whose reproductive development is of current interest to plant breeders. Embryo and endosperm development during the first five days after pollination was studied in plants of 22 genotypes grown at 20 °C with continuous light. Spikes were emasculated before anther dehiscence and then pollinated once full female receptivity was reached. The numbers of embryo and endosperm nuclei or cells in individual florets were ascertained by using large samples of fertilized florets fixed at various known times after pollination. The pattern of early seed development was essentially the same in wheat, rye,
Triticale
and barley, although some interspecific variation in the rate between genotypes was noted. Fertilization occurred in some florets of several genotypes studied within 40-60 min after pollination. Mitosis in the primary endosperm nucleus was completed about 6-7 h after pollination. During the next 24-48 h the number of endosperm nuclei increased geometrically, doubling about every 4-5 h. The endosperm was coenocytic at first but usually at about 72 h after pollination it became cellular. The rate of nuclear development in the endosperm declined on each successive day, the greatest fall occurring at the time of cell wall formation. Mitosis in the zygote occurred about 18-30 h after pollination which was later than mitosis in the primary endosperm nucleus. The cell cycle time in the embryo varied between species from about 12 to 18 h, and was similar to its duration in cells of other meristematic tissues in the same species. Cell cycle time in the embryo remained fairly constant during the first 5 days of seed development unlike the rate of nuclear development in the endosperm. Thus, at first the rate of embryo cell development was very slow compared with that of the endosperm nuclei, however, by the end of the fifth day the cell cycle time in the endosperm had increased to become equal to or longer than that of the cell cycle in embryo cells. The nature and possible cause (s) of rapid nuclear development in coenocytic endosperm is discussed. While embryo volume increased steadily over the period studied, the mean volume of embryo cells decreased about tenfold. This was because at first the rate of increase in embryo volume was lower than the rate of increase in embryo cell number. Eventually these two rates became similar and thereafter further development gave rise to embryo cells whose volume was constant and roughly equivalent to that of other meristematic cells in the same species. The rates of embryo and endosperm development were as a rule much faster in wheat species than in rye. By comparison, the rates in hexaploid
Triticale
genotypes were usually much slower than in wheat, and sometimes even slower than in rye. Results for wheat-rye chromosome addition lines, disomic for each rye chromosome, show that most rye chromosomes apparently had a pronounced effect on slowing both embryo and endosperm development. Indeed, rye chromosomes VI and V II apparently had an effect equal to that of the presence of a whole rye genome. Comparison of the maximum rates of endosperm development in diploid and related polyploid species shows that there was no effect of polyploidy during the first 48 h of the coenocytic phase of endosperm development. Concurrently, during development of the cellular embryo there was a clear effect of ploidy level, with a positive relation between ploidy level and developmental rate. These results are compared with the effects of polyploidy on the rate of development in other tissues in the same species. The rates of embryo and endosperm development in
Hordeum vulgare
were much faster than in diploid
H. bulbosum
. This result is discussed with reference to the mechanism of chromosome elimination from embryo and endosperm tissues of Fj-hybrids between these two species. The present results provide a detailed picture of the course of normal early seed development in a wide range of cereal genotypes which varied with respect to several characters known to affect rate of development in other tissues. They provide, therefore, a baseline for comparative studies which aim both to describe abnormal early seed development and to quantify its extent, in for instance
Triticale
withshrivelled grain. At the same time they provide some indication of the factors which apparently influence or control the rate and extent of early embryo and endosperm development in these important crop species.