Fig. 24.—Section of Raw Popcorn. × 150.—(Courtesy of Bureau of Chemistry.)
Shows cells with the small angular starch grains closely packed together within them.
Argument in favor of the specific claims for these groups is based primarily on the convenience thus attained; secondarily, on the absence or rarity of intermediate or connecting forms, so far as present data extend, and also on the antiquity of the separation. It seems almost certain that in the order of evolution (excluding from consideration the puzzling sweet corn group) progress has been from the pops, through the flints and the dents, to the softs. Certainly the soft corns in some of their varieties present a kernel that is larger, softer, and less fitted to the struggle with natural conditions than is the kernel from any of the other groups. Yet soft corns are the prevailing form in the mummy burials of Peru and of our Southwestern states. The popcorn, on the contrary, has stronger regerminative powers than have the other groups, is better fitted to contend against natural vicissitudes, and is the kind that has been reported as found growing wild in Mexico under the name of Coyote corn, Zea canina Watts.
Some of these subdivisions may not be accepted by botanists, but they are convenient for purposes of description. The principal field varieties which are grown are the flint corn, Zea indurata, and the dent corn, Zea indentata.
POPCORN.
This variety of maize is used very largely in the United States as a delicacy, and with sugar and cream as a dessert. It is a hard, small-grained variety which has the property, when heated, of exploding with a very great enlargement of the starch grain, producing a soft and very delicate edible material which is highly prized.
Fig. 25.—Section of Popcorn in First Stage of Popping, Showing Partially Expanded Starch Grains and Ruptured Cell Walls. × 150.—(Courtesy of Bureau of Chemistry.)
In the raw popcorn the starch grains are packed together very closely within the cells. When popping begins there is an expanding of the starch grains, producing a cavity nearly circular in form in each grain. This causes a rupturing of the cell walls, though fragments are plainly visible in the early stages. In the fully expanded or popped kernel the starch grains have expanded until each is about half or two-thirds as large as the original cells of the endosperm. The cell walls in this stage are practically obliterated as far as detecting in a section is concerned. The exploding of the starch grains is influenced by the water content of the kernel. It must not be too wet nor too dry; about 10 or 12 percent is the proper content of moisture. These changes are beautifully shown in the accompanying microphotographs, [Figs. 24], [25], and [26], by Mr. Howard, of the Bureau of Chemistry.
