Cotton embryogenesis: The tissues of the stigma and style and their relation to the pollen tube

Abstract

The stigma of cotton (Gossypium hirsutum) is covered by unicellular hairs. The cytoplasm of these hairs degenerates before the stigma becomes receptive. The vacuole remains intact, but the hair cytoplasm becomes a mass of dark, amorphous material with only a few organelles still being visible. The rest of the stigma consists of thin-walled parenchyma cells with large vacuoles and large amounts of starch. The cells of the style are differentiated into a uniseriate epidermis, vascular tissue, a cortex of thin-walled, vacuolate parenchyma cells, and the transmitting tissue. This latter tissue occupies the center of the style and consists of thick-walled cells with few vacuoles. The cells are rich in starch, ribosomes, endoplasmic reticulum and dictyosomes. They also contain deposits of calcium salts in the form of druses. The pollen germinates on the stigmatic hairs, grows down the outside of the hair and between the cells of the stigma to the transmitting tissue of the style. There the tubes grow between the walls of the cells but do not enter the cells themselves. Some transmitting cells adjacent to the pollen tube degenerate after the tip of the pollen tube has grown past them. However, not all degenerate, and those that do show no fixed spatial relationship to one another. The cells which do degenerate follow a characteristic pattern of breakdown. No ultrastructural evidence was found for the secretion of hydrolytic enzymes by the pollen tube.