The pollen-stigma interaction in the grasses. 5. Tissue organisation and cytochemistry of the stigma (“silk”) of Zea mays L.

As originally proposed by Raspail in 1824 and endorsed by later workers, the “silk” of Zea mays constitutes a greatly extended stigma, which can be interpreted as being formed by the fusion of the two branches characteristic of the more usual grass stigma. After an early period of cell division in the basal zone, growth in length is mainly by cell extension. The principal pollen-receptive surface is contributed by two irregular marginal zones of trichomes. The trichomes extend throughout the length of the stigma except for a naked zone of c. 5 mm distal to the insertion into the ovary, and in the central part of an extended stigma they occur at a density of 120-140 per cm. The cells of the trichomes are adapted for both internal and external secretion in a manner precisely similar to those of other grasses, and in the receptive state the surface bears a continuous pellicle containing proteinaceous and pectic components. Each trichome arises from a single epidermal cell, and permeability mapping shows that this cell and all derivatives from it bear discontinuous cuticles in the manner of the receptive surfaces of other stigmas of the “dry” type. The morphology of the basal cell-complex of the trichome is such as to provide the initial guidance for the pollen tube in its entry into the principal axis of the stigma. The total pollen receptive area contributed by trichomes of one stigma of the genotype investigated amounted to about 0.19 cm2, an area only some 50% greater than that of the physically much smaller stigma of rye. Two pollen-tube transmitting tracts extend the length of the stigma, adjacent to, but not part of, the two vascular bundles. The transmitting tissue is composed of elongated, fusiform cells, circular in cross section, with abundant intercellular secretion. As in other grass stigmas, this secretion, which forms the pollen-tube transmitting medium, contains proteins and acidic pectic polysaccharides. The tracts lie separated from the marginal trichomes by 5-8 files of cortical cells, with no intervening specialised transmitting tissue.