The ultrastructure of the embryo cells in ovules, from fertilization to the embryo maturity stage in the natural tetraploid Trifolium pratense L. that has a very low rate of seed formation, was examined. Following fertilization the vacuolar organization in the zygote changes. The zygote was a polarized cell and contained a central nucleus, mitochondria, plastids, ribosomes, and lipid bodies. Ribosomal concentration increases significantly after fertilization. The first division of the zygote was transverse or oblique and unequal. The primary endosperm nucleus divides before the zygote nucleus, forming a coenocytic nuclear endosperm; however, part of it later becomes cellular. At the earliest stage of embryo development, the cells were vacuolate, and plastids and mitochondria were simple in structure. During all stages of embryogenesis the suspensor cells were less electron dense than the adjoining embryo cells. Endosperm cellularization begins when the embryo has developed the globular embryo proper. Cellularization starts at the micropylar end of the embryo sac and progresses toward the chalaza! end. Dictyosome activity, ribosomal aggregation, and the amount of rough endoplasmic reticulum were highest during the late globular embryo stage. In addition, the vacuolar volume in the cells was reduced. Lipid bodies were present up to the early globular stage, then disappeared. The inner cell walls of the embryo were thin, with many plasmodesmata. These walls begin to thicken at the late globular stage. The results show a corresponding increase in the amount and activity of the metabolic machinery as the development of the embryo progresses.

In this study, ultrastructural organization of cells in the mature embryo sac of natural tetraploid Trifolium pratense L. was investigated. The mature embryo sac of this plant contains an egg cell with two synergids at the micropylar end, and a central cell with two polar nuclei. The ultrastructure of these cells agrees with what is known for most angiosperms studied with the electron microscope. The egg cell is a large and highly vacuolate cell, partially surrounded by a wall. Much of the cytoplasm is located around the nucleus at the chalazal end and there are few numbers of channel-shaped endoplasmic reticulum, mitochondria, plastids and numerous ribosomes distributed throughout the cytoplasm. Unlike the egg cell, much of the cytoplasm in synergid cells is located at micropylar part of the cell and the synergid cytoplasm contains especially, large numbers of rough endoplasmic reticulum, free ribosomes, mitochondria and plastids. The central cell of T. pratense L. contains two large polar nuclei which lie close to the egg apparatus. Each polar nucleus has a single, large, dense nucleolus that contains several nucleolar vacuoles. Much of the central cell cytoplasm consisting of granular and agranular endoplasmic reticulum, mitochondria, plastids, ribosomes, dictyosomes and lipid bodies are placed around polar nuclei