Popular Science Monthly/Volume 68/January 1906/What Is an Ear of Corn?
|WHAT IS AN EAR OF CORN?|
THE UNIVERSITY OF NEBRASKA
IT is generally thought that corn (Zea mays) originated from some plant like teosinte (Euchlæna), and that the ear is the result of the fusing together of a number of two-rowed pistillate spikelets, such as are found in Euchlæna. Hackel evidently holds this view, for he describes the pistillate flowers of corn as being similar to those of Euchlæna and borne on spikes, except that "the pistillate spikes (originally by monstrous or teratological development?) are grown together into a spongy continuous club-shaped body (the 'cob') upon which the four to eleven double rows (each sessile upon a low longitudinal
elevation, that is limited by a long, shallow furrow on each side) correspond to a single spike of Euchlæna." This view is also accepted by Harshberger, who made a careful study of the corn plant, and I believe is the theory generally accepted as to the origin of the corn ear.
I have often noted abnormal developments or possible reversions which have suggested to me another explanation. These offer much evidence that instead of the ear originating from the fusion of a number of two-rowed spikes, it developed directly from the central spike of some tassel-like structure similar to the well-known corn tassel. Tassels may be found where only a few pistillate flowers have been formed on the central spike and others with more and more such
|Fig 2. A Case in which we have a well developer Twelve-rowed Ear corresponding to the central spike of a tassel and surrounded at the base by a number of tour-rowed 'nubbins' that correspond to the lateral branches.|
flowers, up to where a fair-sized ear has been developed. The accompanying photograph (Fig. 1) shows some of the steps from a normal tassel up to a perfect ear. Note that in the first step the plant is almost normal (at the left), in the second the central spike of the tassel is fairly well developed into a small ear, the number of lateral branches has been somewhat reduced, and the internode below the tassel is somewhat shortened, so that the base of the tassel is partly enclosed. In the third step all the lateral branches have disappeared but two, and the ear-like structure is almost enclosed in the leaf sheaths; in the fourth step only the well developed central spike remains, and is entirely enclosed in the leaf sheaths, owing to the shortening of the internodes
|Fig. 3. a, Central spike of tassel bearing pairs of spikelets in eleven rows; b, lateral branch of tassel bearing pairs of spikelets in two rows; c, tassel in which central spike is developed into an ear.|
below. The last step illustrates a well developed ear, also showing the much shortened internodes below, and the very greatly reduced leaves, which in the highest types of corn completely disappear, leaving only the leaf sheaths enclosing the ear. It is just as possible for the lateral branches to develop pistillate flowers as for the central spike, and they often do so. The accompanying photograph (Fig. 2) exhibits a case in which we have the central spike and also Fig. 4. Staminate Flower of Corn (Zea mays). 1, First empty glume; 2, second empty glume; 3, first flowering glume; 4, first palet; 5, second flowering glume; 6, second palet; 7, lodicules. The upper flower matures first, but the palet and glume are smaller than in the lower flower. the lateral branches developing pistillate flowers. But ordinarily in evolution when one portion begins to develop it is at the expense of other adjacent parts. In such case, the development of the central spike of the tassel is accompanied by the disappearance of the lateral branches. By removing the surrounding 'nubbins' we find that there is a normal ear in the center.
The central spike of the normal tassel usually has from four to eleven rows of spikelets in pairs, making eight to twenty-two rows of corn when developed, while the lateral branches usually have only two rows of spikelets in pairs, making only four rows of grain when well developed.
Fig. 5. Modification of a Pair of Staminate Spikelets into a Pair of Pistillate Spikelets, a, b, c. The pedicellate spikelet shortens down until it becomes sessile, d The sessile flowers become pistillate; e, both flowers become pistillate.
It is interesting to note the morphological changes which take place in the modification of the staminate flower into the pistillate. The staminate spikelets are borne in pairs (sometimes in threes), one sessile, the other pedicellate, the pairs alternating. As already stated, the pairs of spikelets are borne in two rows on the lateral branches, and in four to eleven rows on the central spike of the tassel (Fig. 3). The structure of the staminate spikelet is shown in Fig. 4. The outer glumes enclose two sessile flowers, and are 7-12-nerved; flowering glumes are 3-5-nerved, the palet 2-keeled, lodicules 2, fleshy and truncate. There is usually more or less difference between the upper and lower flowers in a spikelet; the upper flower matures first, and the palet is larger than the glume, while in the lower flower the glume is larger than the palet (Fig. 4).
The first tendency toward the development of a pistillate flower is indicated by a shortening of the pedicellate spikelet until it becomes sessile (Fig. 5, c). This is accompanied by an increased difference between the flowers, as mentioned above. As modification progresses, the lower outer glume shortens and becomes thicker and more corneous. The palet and glume of the upper flower show a tendency to become
|Fig. 6. Specimens from Corn Tassels; a, Branch of pod-corn tassel showing twinned grains; b, Central spike of tassel with tip developed into small ear; c, central spike of tassel developing pistillate flowers near middle; d, same as c with flowers removed to show tendency of central rachis to develop into a cob-like structure where pistillate flowers are borne.|
|Fig. 7. Very Young Pistillate Flower from Tassel of Pod Corn. Note difference between size of pistils and size and form of glumes and palets. The lower flower would probably be abortive.|
|Fig. 8. Hermaphrodite Flower. The parts are all numbered to correspond with Fig. 5.|
|Fig. 9. Hermaphrodite Flower of Maize (Zea mays). The anterior stamen is fairly well-developed, while the other two are mere remnants.|
|Fig. 10. Hermaphrodite Flower of Maize (Zea mays).|
outer glumes are found to be greatly thickened,and somewhat corneous. The palet almost encloses the young ovary, the glume covering only a narrow space on the back (Fig. 8), and the tip of the ovary often protruding. The palet and glume of the lower flower (which is now entirely abortive) are more or less hyaline and closely pressed against the dorsal side of the grain. However, in all varieties of corn both flowers in a spikelet will sometimes be found well developed. Twinned grains are especially common in the tassels of pod corn (Zea tunicata) (Figs. 6 and 7). Sturtevant mentions an ear of podded flint corn from Ohio in which the kernels were twinned in the pod.
Hermaphrodite flowers are sometimes found; in fact, in tassels where pistillate flowers are produced, they are quite common (Figs. 8, 9, 10). The stamens, however, are generally very much reduced or are rudimentary. The dorsal stamen seems to persist longest and will often be well developed, while the other two are rudimentary (Fig. 9). The lodicules are very prominent in the staminate flowers, and will usually be found more or less reduced in hermaphrodite flowers, but they entirely disappear in the pistillate flowers.
Fig. 14. Drawing from photograph of a Sweet Corn Plant to compare with Diagram Fig. 13. Note that the number of nodes in the shortened ear-bearing branches corresponds exactly to the number of nodes in the main stem above point of attachment.
The lower rudimentary flowers may be found in the pistillate flowers of all types of cultivated corn (Fig. 12). The abortive ovary is soon absorbed, but the palet and glume remain to form a part of the 'chaff' on the ordinary corn cob. Fig. 13. Diagram illustrating Probable Structure of Early Progenitor of Corn Plant. The development of the central spike into an ear may now be easily traced. First, the pedicellate spikelet in each pair of spikelets becomes sessile so that we have a pair of sessile spikelets as in Fig. 5, c. Then the upper flower in each spikelet becomes a perfect pistillate flower, while the lower flower in each spikelet becomes an abortive pistillate flower. The pairs of spikelets on the central spike are in four to eleven or more rows, so that by the mere development of the central spike of the tassel into a pistillate spike, we have an eight to twenty-two-rowed ear. This accounts for the well-known fact that corn ears are even rowed.
My observations suggest to me that corn and teosinte may have had a common origin, and that in the process of evolution the cluster of pistillate spikes in teosinte were developed from the lateral branches of a tassel-like structure, while the corn ear developed from the central spike. It is probable that the progenitor of these plants was a large much-branched grass, each branch being terminated by a tassel-like structure, bearing hermaphrodite flowers. Fig. 13 is a diagram of such a plant. As evolution progressed, the central tassel came to produce only staminate flowers, these being higher and in a better position to fertilize the flowers on the lower branches. At the same time, the lateral branches came to produce only pistillate flowers, their position not being favorable as pollen producers, while, on the contrary, they were favorably placed to receive pollen. This differentiation in the flowers was accompanied by a shortening of the internodes of the lateral branches until they were entirely enclosed in the leaf sheaths, as shown in Fig. 1.
Fig. 14 is a sketch of the stalk and ears of a well-developed sweet corn plant after the removal of the leaves and leaf sheaths. It will be noted that the number of nodes in the ear-bearing branches agrees exactly with the number of nodes found in the stalk, above the point of attachment. If these branches were elongated to their normal length, we should have something similar to the diagram in Fig. 13. The lowermost branches usually arise at or below the surface of the soil. They develop their own root systems where they are in contact with the soil, and soon separate from the main plant and become independent plants bearing a proper tassel and ear, in all respects similar to the parent plant. Intermediate between the tassel-bearing branches and the first ear-bearing branches on the main stem there often may be found one or more branches, the tendencies of which seem to be about equally divided between ear-bearing and tassel-bearing, resulting in a structure combining the characteristics of both tassel and ear.
- Hackel, 'The True Grasses' (trans, by Scribner and Southworth), page 38.
- Maize, 'Contributions from the Botanical Laboratories of the University of Pennsylvania, pp. 75-202, 1803.
- Bulletin of the Torrey Botanical Club, 1894, p. 336.
- I wish to acknowledge indebtedness to Dr. Charles E. Bessey for helpful advice during the course of this investigation.