The Journal of Heredity 15(10): 417-419. (1924)
THE ORIGIN OF FLINT AND DENT CORN
D. F. JONES
Connecticut Agricultural Experiment Station. New Haven
EARS OF RICE POP AND CUZCO HYBRID
CROSS SECTION OF GRAIN OF DENT CORN
|Figure 2. The tiny ear on the right is a full-sized ear of California Rice Pop corn The other is a self fertilized ear from a first generation hybrid of Rice Pop with Cuzco. The grains of the hybrid ear are intermediate in size between those of the parental types and contain varying amounts and arrangements of hard and soft starch. Some of the grains resemble dent corn in structure and others contain more hard starch and resemble the flint varieties.||Figure 3. The core of soft starch appears white in the photograph, in contrast to the surrounding layer of hard starch, which is dark. The "dent" is caused by the greater shrinkage of the soft starch when it dries. Grains of flint corn also contain both kinds of starch, but the layer of hard starch extends entirely over the tip of the grain, making a rigid outer shell that does not shrink.|
THE TWO most commonly grown types of Indian corn are the flints and the dents. Both types vary widely in size and form of ears and in the plants which produce these, but each is characterized by having both soft and hard starch stored as food in the seed. Soft or floury starch has plump, smooth starch grains that are easily separated. Areas of soft starch are opaque and when the seeds are colored yellow the soft starch is very pale or colorless. In the hard or corneous starch the grains are cemented together in a dense brittle mass that is more or less translucent and is highly colored by the yellow pigment occurring in yellow varieties. In flint corn the soft starch is restricted to a small area around the embryo and is covered with a thick layer of hard starch. Dent corn has more of the soft starch which extends as a central core to the tip of the seed, the hard starch being placed at the sides. The greater shrinking of this soft starch in the center gives the characteristic indentation of dent corn (See Figure 3).
|*Wallace Publishing Co., Des Moines, Iowa.|
In the small seeded pop corns there is a type in which no soft starch occurs. At the other end of the series we have the soft flour corn of the Indians which has seeds with no hard starch. It is rather easy to surmise that the intermediate dent and flint types have originated from the crossing of these two distinct types of endosperm. There is considerable evidence to support the view that modern dent corn has resulted from the mixing of flint and flour corn as brought out by Wallace and Bressman in their recent book, Corn and Corn Growing.*
A number of quotations from early writers indicate that a late maturing floury type of corn known as "gourd seed" was commonly grown along with the early northern flint varieties in and about Virginia in the early part of the past century, where both the late and early varieties could be grown successfully. Presumably the mixing of these types produced dents of a primitive form and from these the modern types of dent corn may easily have developed.
Crossing Cuzco and Rice Pop Corn
LARGEST AND SMALLEST GRAINS OF MAIZE
|Figure 1. Cuzco corn from Peru undoubtedly produces the largest grains of any variety of maize, and Rice Pop probably the smallest. In internal structure these two types differ almost as much as in size. Cuzco is one of the so-called "flour corns," the grains being made up entirely of soft starch, so that in texture they resemble a piece of chalk. The grains of Rice Pop, on the other hand, contain only hard starch, which is almost like horn in consistency. Crosses between these two divergent types are perfectly fertile, and produce grains resembling both flint and dent corn.|
Both the dent and flint type of kernel can be produced by crossing a completely corneous with a floury variety as shown in the accompanying illustrations. A variety of pop corn called California Rice Pop has the smallest seeds known to the writer to occur in maize. As the most extreme contrast to this stands the large seeded flour corn from the Andes, known as Cuzco. The Rice Pop seeds are from two to three millimeters broad and five to seven millimeters long, the size of a grain of rice, and have no soft starch. The Cuzco has probably the largest seeds of any variety of corn known. They range in size from fifteen to twenty millimeters in breadth and twenty to twenty-five millimeters in length, about the size of a large marble, only flattened. They have no hard starch and in texture are much like pieces of chalk. The larger seeds have about sixty-five times the volume of the smaller seeds. The difference in size of seed is perhaps the greatest that exists in any single plant species.
These two extreme types were crossed both ways. The Cuzco plants were started early in the greenhouse and set in the field at the usual planting time. The plants grew very large, and late in the season produced pollen and one plant ripened seeds which had been fertilized by Rice Pop pollen. These were rather poorly developed and were considerably smaller than typical seeds of this variety grown in Peru due to the shortened season and growing conditions to which it was not adapted. The pop corn seeds from Cuzco pollen developed normally and did not differ from the variety type.
The hybrid plants were started early in the greenhouse and grew vigorously when set in the field. They flowered in good season and ears developed abundantly. These were larger than either parent type, while the seeds were intermediate in size. Several ears were self-pollinated and set full of seeds showing no indications of sterility. One of them is shown in the accompanying illustration in comparison with a typical ear of Rice Pop and an ear of Cuzco grown in Peru (Figures 1 and 2).
Cuzco seeds show a slight tendency to indent due perhaps to their very large size, since smaller seeded floury varieties are perfectly smooth. Many of the seeds on the hybrid ear are well indented. All of the seeds have varying amounts of soft and hard starch and seeds of dent type and flint type can be found on all of the ears. There seems to be no doubt but that fairly good flint and dent types could be separated from this cross.
The Origin of Maize
This direct evidence does not settle the question as to the actual course of events in the production of corn types. In advancing the view that dent and flint corn has originated from the crossing of hard and soft starch types, we cannot be at all certain that the cart has been hitched up the right way. We might as easily assume that flour and pop corn are extreme variations from an intermediate type of somewhat the same nature as our dent and flint varieties. In fact, if Collins' theory as to the hybrid origin of maize is correct, this would be the more logical assumption. Collins gives many cogent arguments in support of a mixed ancestry for maize. One of the assumed parents is teosinte or a closely related ancestral type. The other is a wholly unknown member of a different tribe of grasses, the Andropogoneae, to which the sorghums belong. Many of the sorghums have only soft starch in their seeds. In contrast to this the Poaceae as represented by teosinte (Euchlaena) and gama grass (Tripsacum) have hard corneous seeds with little or no soft starch. On the other hand, some of the small seeded sorghums like Johnson grass have hard corneous grains and broom corn has seeds that will pop like pop corn. The character of the starch stored in the seeds is variable in grasses and the fact that both soft and hard starch occur together in maize is not necessarily attributed to a hybrid origin. But such a beginning accounted for on other grounds might easily bring about an endosperm having varying proportions of the different kinds of starch material. From this all of the known types of normal endosperm could have been derived. Whether the original intermediate condition persisted and finally gave rise to our present day forms of flint and dent corn or whether these have been created anew from the crossing of extreme variations, such as pop and flour corn is largely speculation. The evidence here presented shows that recombination by crossing is a very easy method of bringing about variation in corn from which new forms may be developed.