Development and composition of the spinach ovule

The ultrastructure and histochemistry of the developing spinach ovule have been examined. The development and differentiation of the integuments, nucellus and female gametophyte results in an ortho-amphitropous organisation of the ovule. In the nucellus four parts can be distinguished: the conductive part, the original chalazal part, the chalazal proliferating part and the laterial part. The cells of the various parts have common features as well as distinguishing characteristics related to their position and function. The inner and outer integuments show different features during their development. In the outer integument 3-5 cell layers develop, but the inner integument shows a development of two differing cell layers. Contact by plasmodesmata between the two cell layers diminishes and stops at maturity. The localization of different reserve substances (starch, other polysaccharides, proteins, lipids) have been studied in the developing ovules to determine the nutritional supply of the embryo sac and embryo. Extensive changes in the amount of reserve substances have been observed in the ovule right up to maturity of the embryo sac. In the outer integument storage of starch increases much up to maturity and after fertilization a gradual decrease occurs. The storage and transfer function of some cell types in relation to the nutrition of the embryo sac and embryo is discussed. The presence and location of polysaccharides during megasporogenesis and megagametogenesis is considered. Finally, the possible pathway of metabolites in spinach during different phases of the developing ovule is discussed.