Search

An analysis of the entrance and discharge of the pollen tube into the embryo sac of Gossypium hirsutum was made with the light and electron microscopes. The following sequence of events is seen in cotton:

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.

The “x-bodies” present in the degenerated synergid of cotton were shown to contain DNA by specific staining with Azure B, the Feulgen procedure, and by labelling with 3H-actinomycin D. They are identified on a morphological basis as the degenerated vegetative nucleus of the pollen tube which is always found in the degenerated synergid tip, and as the degenerated synergid nucleus, which is found about halfway up the degenerated synergid near the side towards the central cell. The positions of these nuclei and of some other structures have been used to construct a tentative description of some of the events occurring during pollen-tube discharge and movement of the sperm through the synergid. Some observations which may indicate some of the factors involved in interaction of the synergid and pollen tube cytoplasm are discussed.