Light and electron microscope investigations of the germination of Lactuca sativa L. embryos

The structure of Lactuca sativa Ij. embryos was studied using light and electron microscopy. Dry embryos and seedlings ranging from 12 to 108 hrs. old were examined. Material for light microscopy was fixed in 10% acrolein and embedded in glycol methacrylate. Material for electron microscopy was fixed in glutaraldehyde and post-fixed in osmium tetroxide; it was embedded in araldite-DDSA. In general, cells of dry embryos are filled with reserve proteins and lipids. Nuclei, mitochondria, ribosomes, and some membranes appear to be only slightly changed by their dehydrated state. Plastids and the endoplasmic reticulum are less well-defined and other organelles are absent from the dry embryo. Variations in the structure of the plastids and the protein bodies suggest differences in the degree of dormancy of different parts of the embryo. Reactivation of cells begins just behind the root apex and progresses through the hypocotyl and acropetally in the cotyledons. Changes in the structure of organelles and reserve materials are described. Plastids undergo the most marked changes. Cells of the root tip, hypocotyl, and cotyledons each have a characteristic type of plastids.