J Hered 9: 147-154 ( 1918)
TROPICAL VARIETIES OF MAIZE
Great Diversity of Types of Maize—Most Important Food Crop Cultivated by Natives of America at Time of Discovery—Plant Shows Wide Distribution1
G. N. COLLINS
Bureau of Plant Industry, U. S. Department of Agriculture
1Paper presented before the Second Pan-American Scientific Congress, Washington, D. C., December 27, 1915—January 8, 1916.
MAIZE, or Indian Corn, was the most widely distributed and by far the most important food plant cultivated by the natives of America at the time of the discovery. It is still the most important American crop plant, and while much more productive strains have been developed than any grown by the aborigines, the range of the species on the American Continent has not been greatly extended.
To be able to grow under the widely different conditions that are found in the region from central New England to Argentina and Chile, there must be a great diversity of types. How great this diversity is and how inadequately it has been utilized in developing strains adapted to the needs of the different agricultural communities it is the object of the present paper to discuss. The Department of Agriculture here at Washington has for a number of years been collecting the types of maize from the different parts of the world. These varieties have been grown and their characteristics studied with a view to determining which of the characters they possess may be utilized in developing varieties better adapted to the different parts of the country.
It has not been possible to make an adequate survey of the existing varieties of any country and there are many regions of which we have practically no knowledge of the kinds of maize. Agricultural expeditions that are sent out are usually compelled to pass more or less hurriedly through the countries they visit and the explorers in charge are always charged with collecting a great variety of plants and data, so that little more can be done than to snatch what is exhibited in the markets or what is found growing immediately along their routes. Such collections have been supplemented by seed secured from consuls and correspondents. While many valuable introductions have been secured in this way, ignorance of the normal behavior of the varieties and of the cultural and climatic conditions under which they have been grown often stand in the way of making the most of the peculiar qualities they may possess.
As a result of this rather haphazard manner of securing material our collections are very uneven. In some few places the series of varieties is comparatively complete, but there are whole countries not represented in our collection by a single maize variety. Enough has come to light, however, to show that, especially in the countries of tropical America, we have an immense storehouse of valuable material that awaits the utilization of the breeder.
We have come to believe that the search must be made for characteristics rather than for varieties ready made, as it were. It seldom happens that a variety from one locality will be found adapted to any other locality, but often the most unpromising and insignificant variety, grown perhaps by some backward tribe of natives, will possess some peculiarity or adaptation that needs only to be combined with the desirable qualities of other varieties to become of great value.
Instead, therefore, of attempting to present any complete description of varieties of corn, I shall enumerate a few of the more striking adaptations that have thus far been discovered, beginning with the varieties grown by the Indians of the western United States.
COMPARATIVE ROOT DISTRIBUTION OF MAIZE
|Root distribution of Zea hirta to the left, and of an ordinary dent variety on the right. The surface distribution of the roots of Zea hirta enables this type of maize to utilize the water from light showers and even dew. On the other hand, the failure of its roots to penetrate the soil precludes utilization of stored moisture.|
The Hopi, Navajo, and Zuñi Indians are still growing a type of maize that seems to have spread but little outside the region inhabited by these tribes. The prevailing type of seed is soft or amylaceous, the plants tiller abundantly, they are early maturing, and make comparatively rapid growth at low temperatures. Their exact water requirement has not been determined, but it would appear that they are comparatively economical in the use of water. These characters, while valuable, are all quantitative. This type of maize does, however, possess one unique feature. All other types of maize thus far studied produce three or more roots from the seed. These roots support the young plant until the permanent roots that develop from the nodes have been formed. In the type under discussion there is but one root developed from the seed. The economy of this arrangement seems to be that the single root is able to penetrate to a greater depth than is possible when the reserve material of the seed is divided among three or more roots.
In combination with this character there is also the possibility of a much greater development of the mesocotyl than in other types. The mesocotyl is a specialized organ found in the seedlings of maize and some other grasses. It may be described as the part of the main axis that connects the seed with the first leaf of the seedling. So long as the germinating seed remains in the dark, the mesocotyl elongates, stopping only when the tip of the seedling reaches the surface of the ground or the limit of elongation is reached. In planting their maize the Hopi Indians dig into the sandy soil until they reach moist earth, usually found at a depth of from 6 to 18 inches. The seed is deposited in these holes in contact with the moist soil. Although these holes may be left only partly filled, the drifting sand soon fills them, and to reach the surface the young plants must penetrate from 6 inches to a foot of soil.
Experiments have shown that commercial varieties of maize planted under these conditions will not reach the surface. The maximum development of the mesocotyl in commercial varieties is from 8 to 10 centimeters. After this elongation is reached the leaves attempt to expand beneath the ground, and as they unfold they are not sufficiently rigid to force their way through the soil.
It was found that the mesocotyl of Hopi maize is capable of elongating to a length of 36 centimeters or over three times the length attainable in other varieties. At the same time the single strong seminal root is following the retreating moisture and keeping the young plants alive until the summer rains wet the soil above and allow the development of lateral roots. A long mesocotyl thus divides the work of reaching moisture with the single seminal root and the combination makes possible the establishing of young plants in soils where the nearest available moisture is a foot or more below the surface of the ground.
These peculiarities make this type of maize beautifully adapted to the conditions that obtain on the wind-swept plateaus of northern New Mexico and Arizona where the variety is grown. In this region there is a winter rainfall, but by the time the weather is sufficiently warm to warrant the planting of maize, the surface soil is thoroughly dry. Summer rains may be expected in June or July, but if planting were delayed until the rains came there would not be time enough for the crop to mature before the frosts came in September.
These adaptations should find an application wherever maize is planted during a dry season and the young plants are forced to depend on moisture stored in soil. At San Diego, Cal., the past season plants of this type matured normally without a drop of rain during the growing season.
That adaptations of so great economic importance should exist inside our own country and remain unnoticed is a striking indication of how inadequately the possibilities of maize development have been investigated.
Another set of interesting adaptations have been found in a type of maize from the table-lands of Mexico. This type was early recognized as a distinct form. In 1829 it was given specific rank by Bonafous under the name Zea hirta.
The most striking characteristic of this type is a peculiar development of hairs on the leaf sheaths and also to some extent on the blades. For this most conspicuous peculiarity we have not as yet been able to discover any adaptive significance.
On the tablelands of Mexico where this maize is grown the rainfall is very light. In the parts of the United States with a similar rainfall maize production is not considered possible. We were, therefore, extremely optimistic regarding the utilization of this type of maize. Our early experiments were very disappointing. In the semiarid regions of the West this variety was a complete failure. Instead of being drought resistant the plants appeared to suffer from drought more than the ordinary varieties from the corn belt. Without the viewpoint that each distinct type of maize possesses adaptive characters this variety would have been discarded as worthless. As soon, however, as the variety was scrutinized with respect to the separate characteristics instead of trying to utilize it as it was, important adaptations came to light. The first peculiarity noticed was the nature of the root distribution. In this type of maize the roots seem to have lost the ability to penetrate the soil. In a full grown plant the roots are all horizontal and confined to the upper 6 inches of soil. There is so little direct attachment to the soil that the whole plant can be lifted up and down by the hand. The plants are on rather than in the ground. This peculiar root distribution helps to explain why the plants are able to grow in Mexico with so little rainfall and why they fail in the western part of our country. In the part of Mexico from which this variety was secured the little rain that falls comes during the growing season in the form of light, misty showers. The rainfall is at no time sufficient to penetrate to any extent and an extensive superficial root system is best adapted to utilize the moisture.
This particular adaptation we are unable to utilize in this country, for in the drier parts of the United States the rainfall comes largely in the winter instead of during the growing season, and growing plants must draw their water from that stored in the ground some distance below the surface. There are, however, many regions in tropical America where this adaptation should be of value. Through the work of Briggs and Shantz, of the Department of Agriculture, it has been shown also that Zea hirta is the most economical of water of any of the varieties of maize yet studied.
Still another adaptation possessed by this type of maize was observed during the past season in the course of experiments conducted near San Diego, Cal. It there developed that this type of maize made satisfactory growth at lower temperatures than any of the other types with which we were experimenting. This does not mean frost resistance, nor does it mean that this type is suited to extend maize growing farther north. It means rather that this type possesses one of the characteristics necessary to a variety of maize for regions where the temperatures are uniformly too low for the ordinary varieties. Temperatures below the optimum for maize are the rule over a large part of the elevated regions of the tropics. In the development of varieties for cool climates, this type of maize promises to be of great value. Our experience with Zea hirta shows also the folly of looking for drought resistance as such. To make progress, we must distinguish between the different kinds of drought resistance and search for the particular adaptation needed.
Nearly every region from which we have received varieties has contributed adaptations that promise to be of value. From Bolivia has come a type possessing to a marked degree the quality of remaining green for a long time after maturity. Mexico has given us one variety with the largest ears and another with the ability to withstand extremely high temperatures. Peru contributes the largest seeds. From China comes a variety with a new type of endosperm and the ability to withstand hot dry winds at the time of flowering. Our best protected ears come from Guatemala.
Enough has now been said to indicate what is meant by adaptation and how the varieties must be studied to realize and appreciate the adaptations. Only a beginning has been made. When it is realized that no two regions present exactly the same environment and that maize, though a very ancient crop, is very plastic and has molded itself to the conditions under which it has been grown, some idea may be gained of the multitude of adaptations that await discovery.
MAIZE FROM PERU AND BOLIVIA
|Seeds of Cuzco maize from Peru and an ear of a Bolivian variety of maize having the quality of remaining green a long time after maturity.|
RECOMBINATION OF CHARACTERS
A word must now be said regarding the recombination of these adaptations into new varieties suited to new conditions or representing an improvement over varieties already existing.
Regarding the recombination of characters, our knowledge is still very imperfect, but here also a beginning has been made and results sufficient to serve as demonstrations have been secured. An example may be taken that is particularly applicable to tropical America.
When a traveler from the north visits tropical America and finds maize a staple crop and often growing luxuriantly, he is naturally struck by the absence of table or sweet varieties so generally grown in this country. I have made inquiry in a number of instances and almost invariably have been told that sweet varieties have been introduced, but that they did not do well, and that their growth was given up on that account. If it were possible to combine the palatability of the sweet varieties with the luxuriant growth and freedom from insect attack of the native varieties, a valuable addition might be made to the rather meager list of really delicious vegetables available in many parts of the tropics. We are to some extent confronted with the same problem in our own southern states where the ravages of the corn worm practically preclude the growing of commercial varieties of sweet corn.
In the breeding of commercial varieties of sweet corn, one of the most important considerations has been earliness. In breeding for earliness the number of leaves has been reduced. In reducing the number of leaves the number of husks, which are homologous to leaves, have also been reduced with the result that the ears of sweet corn are poorly protected. This was of little or no importance north of the region infested by the corn worm, but it is this that renders the commercial sweet varieties unsuited to southern regions.
In 1912 crosses were made between commercial sweet varieties and southern varieties of field corn having well protected ears. The first generation plants were grown in 1913, and selections were made from those plants with the most perfectly protected ears. From the ears thus obtained, which contained a mixture of sweet and non-sweet seed. We selected the sweet seed for planting in 1914. In that season the plants were very variable, but all the seeds were sweet. Crosses were made between the most promising plants, special attention being again paid to the covering of the ears. The results of the past season demonstrated that we already have a fairly uniform and productive variety of sweet corn. Although grown in a region where the infestation of corn worms is particularly severe the damage to the cars was insignificant, less, in fact, than was the damage done to field varieties in the same region. Thus in three years we have combined the sweet seeds of the table varieties with well-protected ears of the larger field varieties. It would appear that there is no valid reason why any region that can grow maize successfully should be without sweet varieties. Whether earliness must necessarily be sacrificed, or whether it is possible to secure earliness and still keep the well-protected ears, is an interesting question that must await further investigation.
All characteristics cannot be manipulated as easily as the sweet endosperm, which from the alternative nature of its inheritance permits of Mendelian recombination, but our experiments indicate that most of the characters of maize are independent of one another in inheritance and that recombination is only a question of careful selection following hybridization.
WAXY CHINESE MAIZE
|A typical plant of the Waxy Chinese variety of maize, showing numerous tassel branches, erect leaf blades and curved tassel. The kernels of thie remarkable Chinese maize have a waxy endosperm which distinguishes them in a striking way from other varieties of maize. Photograph by courtesy of the Journal of Agricultural Research.|
The work of discovering new and valuable characteristics is one in which a cooperative arrangement among the different maize-growing countries of America would be especially applicable. Once an adaptation is observed and the discovery announced, it becomes available to all countries.
Of course, an important step to take is the interchange of varieties, but it is of equal importance to have the varieties studied in their native countries by observers familiar with the maize plant and its variations.
I would strongly urge that each of the maize-growing countries of America make a canvass of the maize varieties existing within its boundaries. In the study of these varieties the particular conditions under which they have developed should be kept in mind. Thus, if a variety is found growing in a region of cool nights, high winds, or any other pronounced environmental factor an effort should be made to determine how the variety meets the peculiar condition. I venture again to call attention to the necessity of looking for desirable adaptations rather than desirable varieties and to urge that no variety be overlooked simply because it appears insignificant or worthless. If a variety is growing under climatic conditions that are extreme in any particular, it is more than probable that the variety possesses valuable characteristics.
A HAIRY VARIETY OF MAIZE
|Leaf sheaths of the Esperanza variety of maize, showing the maximum development of tuberculate hairs.
Photograph by courtesy of the Journal of Agricultural Research.
See also — Collins: Pueblo Indian Maize Breeding (1914)