A histological and histochemical study of the cotyledons of Phaseolus vulgaris L. during germination.

Abstract

The cotyledon ofPhaseolus vulgaris L. comprises four tissues: epidermis, abaxial hypodermis, storage parenchyma, and procambium. A complex intercellular space system is present throughout the storage tissue and comprises about 16% of the cotyledon volume. All the cells contain protein bodies, and the hypodermis and storage parenchyma also contain starch grains. The epidermal cells are at the 2 C level of DNA, those of the hypodermis at the 4 C level, and the storage cells vary from 8 C to 32 C. During germination stomata differentiate in the epidermis. Reserve mobilization begins in the cells furthest from the epidermis and from the vascular tissue. Protein is removed from these cells with little or no coalescence of protein bodies. The DNA content of the nuclei decreases. The cell walls swell and then decrease in thickness as material is removed. Finally the nuclei and cytoplasm disappear and the cells collapse. In the cells near vascular bundles the protein bodies coalesce before losing their protein. The DNA content of the nuclei declines but nuclei and cytoplasm are still present at abscission. These cells do not collapse. Cytoplasmic RNA content is highest near the abaxial surface. Most of the RNA is removed during the first three days of germination.