SEGREGATION FOR THE WAXY CHARACTER IN MAIZE POLLEN AND DIFFERENTIAL DEVELOPMENT OF THE MALE GAMETOPHYTE

Abstract

Random fertilization among compatible gametes is one of the cardinal principles upon which the Mendelian theory of inheritance turns. The array of breeding facts on which the concept rests is so extensive, and the approximations to expectation on the chance basis are so satisfactory in almost innumerable cases, that the general validity of the rule would seem to be established. The fusion of male and female nuclei is associated, however, with various morphological features promoting the sexual act. It has been thought that in certain forms these features may impose conditions upon the central feature of fertilization tending to disturb the chance relation. The mechanism attending fertilization in the angiosperms is a case in point. 2 The events leading to fecundation in the angiosperms are unique in that the two sperms contributed by each microgametophyte are dependent for their transference to the embryo sac upon a pollen tube whose development is governed by a third member, the vegetative or tube nucleus. In its development the pollen tube traverses the style, a commonly elongated structure connecting the receptive surface with the ovary. The tube nucleus is identical in its genetic constitution with the sperm nuclei accompanying it, but takes no part presumably in fertilization proper; but, as is evidenced by its larger size and its position in the most active region, it dominates the metabolic processes of the gametophyte. In contradistinction to the passive sperm nuclei, the vegetative nucleus becomes the centre of the brisk activity characterizing pollen-tube growth. Putting it in figurative terms, the tube nucleus pilots the conveyance in which the gametes are passengers. This mechanism interposed between segregation and fertilization might offer a basis for some selective action through differential pollen-tube growth if it were a fact that the microspore nucleus and the tube nucleus during their activity realized their hereditary potentialities to a significant degree. There is evidence from a few forms that this is the case. Belling I has