More effort is necessary to understand the type and distribution of the vascular system in Zea in relation to the origin of the maize cob, to the expression of genes affecting the cob and to the development of energy sinks. A double vascular system was described in the cob as two independent systems (Laubengayer, 1948, 1949; Reeves, 1950). A small type of bundle occurs in the cupule wings and rind, while a larger type is found near the margins of the pith, rarely in the center. Connections between the two systems were discovered later in the glume-cushion (Galinat, 1959). In describing the vascular anatomy of a four-rowed ear of maize that was apparently a mutation out of eight-rowed maize, Laubengayer (1948) observed that because the barren sides without spikelets had the same inner and outer systems corresponding to eight-rowed maize, a reduction in ranking had occurred in this case.
Apparently, genes that alter ranking operate independently from those that change vascular development, although selection tends to produce genetic combinations that are balanced either for teosinte in the wild or maize under domestication. Such changes in one or more genes that are coadaptive with changes in other genes are part of the evolutionary process. Another example within maize is genetic increases in husk length that became necessary for bird and insect protection following genetic increases in ear length.
In the teosinte fruitcase, vascular development is primarily in the outer bundles of the rind, while in maize the shift is to the inner bundles of the pith in proportion to increases in kernel size and row number. This reflects the importance of channeling photosynthate into induration of the teosinte fruit case, in contrast to supplying photosynthate for kernel enlargement and increased endosperm storage in maize. In maize with large hard kernels, there is an adaptive need for an induration of the cupules in order to prevent cob shrinkage during drying and, thereby, continue to provide adequate kernel space.
Walton C. Galinat
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