A Comparative Developmental Study of a Dwarf Mutant in Maize, and its Bearing on the Interpretation of Tassel and Ear Structure

The homologies of tassel and ear structure in maize, and the problem of the morphological relationship between the inflorescences of maize and other genera in the Gramineae, have provided a fascinating study for many workers. Mangelsdorf (1945) has lately compiled a critical review of the earlier contributions to these problems, and with the aid of more recent data has endeavored to formulate a hypothesis incorporating the main features of previous theories. It is interesting to note that most workers have considered the maize inflorescence (tassel or ear) as a unit for comparative study which is complete in itself. Anderson (1944) first realized the importance of considering the structure of the inflorescence as a culmination of developmental processes which are common to the plant as a whole. It would seem that a study of the inflorescence apart from the rest of the shoot might neglect a very valuable source of information, and that a better understanding of the general developmental plan in the maize plant might furnish a new line of attack in interpreting the particular morphology of the inflorescence. Adopting this line of approach, the problem of ear and tassel homologies involves three ontogenetic questions: (1) Bonnett (1940) showed that the primordia giving rise to lateral branches, and the primordia giving rise to paired spikelets on the main axis of the tassel, are homologous structures. Paired spikelets may therefore be regarded as greatly reduced lateral branches. Is this reduction peculiar to the tassel or is it the culmination of a process initiated during the vegetative phase? (2) Mangelsdorf (1945) concluded that the ear of maize as exemplified by the female inflorescence of Guarany pod corn is "fundamentally identical with the spikes of certain other species of grasses .... and like many of them is probably derived from a panicle as the result of reduction of branches. There is not the slightest evidence of fusion." Without in any way disagreeing with the first part of this statement, it may be pointed out that from the developmental point of view the morphology of the panicle represents the crux of the problem; namely, by what developmental process is the simple alternate (distichous) arrangement of primordia, as seen in the basal vegetative portion of the shoot, transformed into the polystichous arrangement found in the apical reproductive region. (3) Bonnett's developmental studies have shown that the florets of both male and female inflorescences are potentially hermaphrodite. What factors determine