A Morphological Analysis of Row Number in Maize

The kernels of an ear of maize are usually set in longitudinal rows of from 8 to 30 or more. Certain genetic stocks, and an occasional ear in open-pollinated varieties, may have only 4 rows. For numbers much above 30, the kernels are so crowded that the rowing is obscure; various irregularities may modify or conceal the row pattern (Weatherwax, 1916, 1917), but in most of the varieties of Zea Mays the kernels over a good portion of the ear are arranged in definite and usually conspicuous rows. In nearly all varieties the row number is even, since the kernels arise from paired spikelets which are themselves disposed in regular rows (Collins, 1919). The history of row number is apparently complex. Both in South America and in the southwestern United States archaeological investigation has demonstrated that the earliest varieties had 12 or 14 rows (Bird and Anderson, in press), and Bird's recent excavations indicate that this condition persisted unchanged for a long period. Both in South America and in the southwest 8and 10-rowed varieties appeared at a later date (Carter and Anderson, 1945). Another type of change in row number has been associated with the dent corns of Mexico. Since at least the times of the Toltecs, row numbers of 16 and above have been characteristic of central Mexico (Anderson, 1946), and it is apparently chiefly from that center that they have been so widely spread around the world as to characterize many of the world's centers of commercial corn production. Since the early days of studies upon inheritance in Zea Mays (Emerson and East, 1913) it has been apparent that the genetic basis for differences in row number is complex. Extensive and careful studies were carried on by the late R. A. Emerson and his students and collaborators for several decades. While he made a number of preliminary and informal reports his results in different crosses were rather contradictory and have not yet been formally published. Lindstrom (1929, 1931) observed linkage between genes for row number and genes for pericarp color. The morphological bases for differences in row number are difficult to study upon the ear itself, an organ so modified that its exact relation to other grass inflorescences is still a subject for research and so highly vascularized that such research is technically difficult. Accordingly, for the past decade, we have carried on a comprehensive survey of variation in both the ear and the tassel (the male and female inflorescences). As Bonnett's (1940) developmental studies have clearly shown, these two inflorescences are scarcely distinguishable morphologically in their early stages but beyond a certain point the ear becomes progressively harder and thicker, the tassel progressively more lax and expanded. From modern