Data were obtained and tabulated on seeds, endosperms, and embryos from P. ponderosa in a wide range of sizes from a tree previously described in Trans. Ill. Acad. Sci. 38. Volumetric measurements (mm3) were made on seeds and endosperms, cotyledons counted, and embryos measured; their volumes computed from diameters and lengths; their values compared in seeds of various size classes. Mean growth rates of embryos were obtained and represented in growth curve[long dash]fertilization to ripeness of seeds. For a period of about 5 weeks after fertilization, there is little embryonic differentiation while numerous embryos within each seed are engaged in a competitive struggle for survival[long dash]the period of embryonic selection. After the dominant embryo has been selected it spurts ahead in rapid growth, differentiating organs in a period of 10-12 days. Seed ripening occurs in a few weeks with little change in embryo size. A great diversity of stages is found among embryos collected on a given date. This makes possible a computation of the presumable rate of growth between successive collections of seeds from the same tree. During differentiation, the av. growth rate is about 1/2 stage per day. It is somewhat slower in the largest, much slower in the smallest seeds. At the time of shoot-tip differentiation and cotyledon initiation, the vol. of the embryo is closely related to the vol. of endosperm and seed. There exists a correlation among seed size, embryo size, and number of cotyledons.