Fine Structure of Early Endosperm in Quercus gambelii

Abstract

This study follows the ultrastructural changes that take place in endosperm tissue during the early stages of embryo development in Quercus gambelli; i.e. from the two-celled embryo through the heart stage. Although the degree of change varies, similar sequences in endosperm transformations are present at all stages examined. The formation of cytoplasmic islands by a pinching off action of the tonoplast is characteristic of early transformation. The endoplasmic reticulum (ER) changes from the rough, lamellate type in early stages to an entirely vesiculate type until it finally disappears from degenerating cells. The vesiculate ER cisternae are usually lined with ribosomes and often contain various osmiophilic inclusions. Plastids are numerous and highly variable in shape. They are characterized by the presence of osmiophilic bodies, and rarely contain starch. Lipid bodies are present at all stages, but never in large quantities. Mitochondria lack osmiophilic bodies; in degenerating cells they are surrounded by osmiophilic deposits, and the cristae lose their integrity. Polysomes are present in all stages examined, except in degenerating cells where the cytoplasm becomes so dense that ribosomes and other cytoplasmic components are indiscernible. The developing cell wall has many discontinuities in section view. Numerous plasmodesmata are present, but they disappear from the common wall between degenerating cells. Dictyosomes appear to be associated with cell wall growth; they disappear in degenerating cells. The plasmalemma in degenerating endosperm cells becomes distorted and disrupted at places, but the tonoplast appears intact. Although various possibilities are discussed, it is concluded that the endosperm of Q. gambelii does not have a major function as a food storage tissue (as evi-denced by the low amount of starch and lipids) but rather, it serves as a translocating tissue between the food source (stored food within the outer integument, and food coming into the ovule through the vascular supply) and the embryo. The cotyledons, thereby, become the storage area within the developing and mature seed.