Proc. Am. Soc. Hort. Sci. 41: 398-411 (1942)
Influence of the Environment on the Expression of Hereditary Factors in Relation to Plant Breeding1
S. H. Yarnell,
Texas Agricultural Experiment Station,
A. and M. College of Texas, College Station, Tex.

1Address of the retiring chairman of the Southern Section of the American Society for Horticultural Science, given at the Memphis meeting, February 7, 1942

ENVIRONMENTAL influence on the expression of hereditary factors ha many aspects. To the geneticist this is the cause of non-hereditary variation. To the experimental taxonomist it helps to explain the status of geographic races. To the breeder it may represent the opportunity to provide adaptability. To the horticulturist interested in cultural problems, differential varietal response to the environment is being increasingly recognized as an important factor in making cultural recommendations. Each aspect has as its fundamental basis the response of the hereditary factors or genes, either singly or more commonly in groups, to the many conditions external to the organism, under which it develops. These outside influences are usually rather complex and difficult to control experimentally. Those most frequently studied are temperature, light intensity and duration, soil and air moisture, wind movement and a variety of nutritional factors. This obviously is more than can be encompassed in detail in a single discussion. For this reason only enough work will be reviewed to indicate their bearing on plant breeding in the South.

One of the first careful studies of the effect of the environment on the expression of the gene was made by T. H. Morgan (50) and reported by him in 1915. A strain of Drosophila was found in which the abdomen was defective. This was shown to be sex-linked and to be due to a single mendelian factor. The remarkable thing about it was that only the flies that hatched while the colony was young had the defect, while flies emerging later were normal in every respect. By suitable tests it was shown that flies developing from larvae whose food had been moist had the defective abdomen, while flies whose larval stage was spent on drier food were normal. Much later Braun (10) discovered another strain that developed an abnormal abdomen on dry media, while flies from larvae on moist food were normal. These instances illustrate two points that should receive emphasis. (a) The expression of a gene of known genetic behavior can be influenced by the environment, in this case moisture, to produce adults differing widely in appearance, and (b) under appropriate conditions of the environment the individuals of a special genetic strain can not be distinguished from the type taken as normal.

A second component of the environment that has received considerable attention from the geneticists working with fruit flies is temperature. Zeleny (86) in 1923 showed that an increase of 1 degree C during the larval stage decreases the number of facets of bar eye by 10 per cent, of ultra-bar by 8 per cent and of normal flies by 2.5 per cent. He pointed out that the same effect can be obtained either by increasing the temperature or by adding another bar gene. Surrarrer (72) secured an increase in mottling in a mottle-eye stock by decreasing the temperature from 24 to 18 degrees C. Flies developing at 24 degrees or above are normal with respect to this character.

The effect of temperature on the development of various bristles has been studied by several investigators. Plunkett (58) found that in general an increase in temperature results in a reduction in the number of bristles. He considered that the effect is due to the acceleration of a bristle inhibitor with increased temperature at a greater rate than the normal bristle former. Child (14, 15) discovered that an increase in temperature decreases the mean number of some bristles but increases the number of others. The effect of temperature, then, depends upon the genotype. Each type has its optimum temperature for bristle formation. A change of temperature gives a differential effect on the rate of reaction of the various gene-controlled bristle forming and bristle inhibiting substances.

A number of genes for defective wings of Drosophila have been studied. Harnly (32) found that the effective temperature for increasing the wing length of vestigial is between 30 and 32 degrees C. Eker (19) states that flies homozygous for genes for short-wing are normal at 14 degrees but have typical short wings at 27.5 degrees. This factor also affects the size of the eye. Jennings (39) mentions a gene in Drosophila for extra legs at cool temperatures.

Passing on to another insect, the Hymenopterous parasite of the oriental fruit moth, Trichogramma minutum  consists of several morphologically similar races that carry different factors for body color which are entirely dependent upon an appropriate temperature for their expression. According to Peterson (57) when the average daily temperature exceeds 62 degrees F the females of one race have a distinct lemon yellow body color, but when the temperature drops below this average these same individuals become a metallic brown. Flanders (23) finds that four races of this parasite can be identified on a basis of color when raised at the same temperature, but when raised at different appropriate temperatures they are indistinguishable. Temperature with this insect not only affects body color but also influences the length of the life cycle, an important character in determining adaptability to climate.

This whole problem of the role played by temperature in the expression of the genes and gene combinations in insects has been given especial attention by Goldschmidt (28, 30) who has developed a theory of gene action through physiological processes that may be hastened or retarded differentially at various temperatures. He has been able to produce intersexes in the gypsy moth in this way, and he has modified the appearance of certain butterflies to correspond to various geographic forms. He lists no less than 14 known genetic characters of Drosophila that can be induced by suitable heat treatment of the normal larvae.

The effect of temperature on the expression of hereditary factors is by no means limited to insects. In Canna, Honing (37) finds that the anthocyanin pigment producing "old purple" is recessive to another factor for yellow. Plants that are homozygous recessive for old purple and heterozygous for yellow (ssWw) are completely yellow during the heat of summer, but later flowers of the same plant developing during cool weather in the fall have a bluish cast. Primula sinensis has a form in which the flower is red at 20 degrees C, but white at 30 degrees. A genetically distinct type has a white flower at both temperatures. Barnes (6) and also Magruder and his associates (43) report that the carotene content of carrots develops best between 60 and 70 degrees F, less color developing either above or below this optimum range. R. C. Thompson (76) secured more intense anthocyanin color in lettuce at temperatures below 50 degrees. In the case of the tomato fruit Vogele (83) gives 24 degrees C as the optimum temperature for the formation of lycopene. Fruits developing at 32 to 38 degrees are bright yellow and they remain green at 40 degrees. Work in Texas and elsewhere suggests that factors exist in the tomato that permit the normal development of red color at temperatures as high as 35 degrees.

H. C. Thompson (73, 74) has studied the influence of temperature on flowering. He secured differential response of celery varieties subjected to temperatures below 50 degrees F to induce seed stock formation. The effect of low temperature on the flowering of biennials depends upon (a) the temperature employed. (b) length of time exposed, (c) stage of development, (d) kind of plant, (e) photoperiod. and (f) later growing conditions. All of us are familiar with the necessity of many fruits and ornamentals for cold during the dormant period in order to bloom and fruit normally in the spring. Varieties differ widely in their cold requirements. The use of cold in vernalization treatments to hasten fruiting would seem to be a variation of this same theme.

The bearing of these differential responses of plant varieties to heat or to cold on the problems of plant breeding may not be immediately evident to those who handle plants in an environment for which their crops have long been bred or selected. Once these plants are grown outside of their accustomed environment the necessity for hereditary factors that will permit them to grow and produce as an economic crop are soon apparent.

The same may be said of the responses of plants to light. The literature concerning the reactions of plants to photoperiod has become rather extensive. The interest of southern plant breeders in this subject arises from the shortness of the days experienced during both summer and winter compared to the day-length of the normal growing season farther north where many of our commercial varieties were developed. Allard and Garner (1, 26, 27) in joint reports have shown that some plants require a relatively long day to flower, others need a fairly short day and still others seem to be relatively independent of this influence. These authors suggest that the lack of flowering observed in numerous introduced plants may be due to an unfavorable photoperiod. The manipulation of the length of day is sometimes desirable to bring two types of plants into flower at the same time in order to make a cross, as was done by Emerson (20) with teosinte.

Sometimes other types of development affected by the light period are of economic importance. For example, only four varieties of onions will bulb properly during the short day of the extensive commercial onion areas of Texas. Both McClelland (46) and Magruder and Allard (42) have found that only Bermuda will bulb satisfactorily with 12 hours of daylight. The work of Breslavec (11), McPhee (49), Schaffner (69, 70) and others on hemp shows that length of day has an important bearing on sex expression. Intersexes and sex reversal can be induced in hemp by providing a short day. A similar effect was obtained by Richey and Sprague (63) with corn. Several inbred strains responded differently to changes in day-length. Allen (2) points out that "environmental factors can influence the expression only of potentialities genetically present. If a once pistillate plant is induced to produce staminate flowers, the results show that genetic factors are present which make possible the production of stamens."

The effect of photoperiod is often conditioned by temperature. Knott (41) found that cooler early temperature and warmer late temperature with a 15-hour day favored seed stalk formation in spinach. Either continued low temperature or a 7-hour photoperiod restricted blossoming. Lettuce heads best at 60 to 70 degrees F according to Thompson and Knott (75) and will not head from 70 to 80 degrees even with a short photoperiod. Roberts and Struckmeyer (64, 65) find that temperatures above and below the normal change the photoperiodic response of many plants. Poinsettia blossomed normally for them in the winter at 60 to 65 degrees but did not at 68 to 70 degrees, or between 55 and 57 degrees. In the summer it bloomed at the last named temperature.

They (66, 67) later report that certain seedling populations of petunia and other plants gave a uniform response with respect to growth and flowering under one temperature and photoperiod but showed "segregation" under other conditions. They ascribe this to genetic differences that are not apparent under one set of conditions but do appear tinder other conditions. Since varietal differences of this kind are common, there seems to be no a priori reason why this explanation might not be accepted. In fact this falls directly in line with other work. The only difficulty in accepting it lies in the fact that similar "segregation" was observed among cuttings obtained from the parent of the seedlings. It may be that differences in the initial size of the cuttings and some lack of uniformity in their handling resulted in variability among the cuttings grown under abnormal conditions of temperature and photoperiod that would simulate differences due to genetic diversity. Further work would seem to be required to establish these assumptions.

Often environmental factors other than or in addition to temperature and light make a significant contribution to the appearance or behavior of a plant. Brainerd and Peitersen (9) found that light intensity, humidity, altitude and moisture supply greatly affect the characters of wild blackberries that have been widely used in their classification. For example, the leafy bracts and prickles develop much better in a cool moist shady place than in full sun. The pungency of onions is another such character. Platenius and Knott (58) found that onions on peat are twice as pungent as those grown on sand, and onions on loam are intermediate in this respect. Pungency tends to increase with increased temperature. More sulfur in the soil increases pungency but more soil moisture decreases it. In spite of these environmental effects, with comparable conditions, some varieties are three times as pungent as others.

Available sugars inside the plant are an important factor in the formation of anthocyanin pigmentation. Environmental factors influencing the accumulation of sugars therefore affect plant color. These include soil nutrients, light, temperature, water, available nitrogen and altitude. Owen (54) reports increased mottling of soy bean seeds on heavy, rich soil, and sometimes with wider spacing, while increased nitrogen decreases mottling. A similar situation has been found with respect to the amount of pigment in the seed coat of field beans (55). Ratsek (62) has been able to reduce the intensity of color of red roses to nearly white by defoliation and by pruning away carbohydrate reserves.

Work on the manipulation of a multitude of cultural factors has now assumed enormous proportions. It is all based on the assumption that the complex of hereditary factors affecting growth and yield are influenced in their expression to a considerable degree by the environment. The differential response of varieties to identical treatments has been mentioned. Attention is called to a few cases among both plants and animals where a change of nutrition has affected a single character or some otherwise striking result has been obtained.

To return to Drosophila, the vermilion brown stock lacks the amount of eye pigment of normal flies. When the 70-hour larvae are placed on a partial starvation diet the intensity of color is greatly increased. It is estimated that this treatment stimulates the production of the eye color hormone by "not less than a hundredfold" (7). In studying a stock of the bent nose Norway rat in which about half of the individuals had this defect when fed a home made diet, Heston (35) was surprised to get only normal rats when the stock was placed on Purina Fox Chow. It was found that a certain calcium-phosphorus ratio and vitamin D do not allow genes for bent nose in the rat to express themselves.

Work on the effect of nutrients on the fruiting of plants has produced rather striking results. Many fungi, as you know, are induced to produce sexual spores on artificial media only with great difficulty or not at all. Sax (68) grew field beans with white and with colored seed coats on rich and on poor soil. Factors linked with color gave the higher yield under unfavorable conditions, while factors linked with white seed gave a higher yield under favorable conditions. Similar results were secured by Poehlman (60) with two varieties of soybeans. The Morse variety outyields Virginia on fertile soils and good growing conditions, while Virginia produces more than Morse under less favorable conditions.

The effect of differences in soil reaction is sometimes rather striking; for example the blue or pink flower color in Hydrangea. Buxton and Darhishire (13) have investigated the behavior of anthocyanins at different hydrogen-ion concentrations. There are two main groups. The blue group is "lake red" at pH 3 and blue at pH 7. The red group is vermilion red at pH 3 and changes to a brownish purple at higher concentrations. Purple or magenta flowers contain both types.

A somewhat different type of effect of environment on gene expression is found in its influence on disease resistance. Walker and Smith (84) report a decreased resistance of commercial varieties of cabbage resistant to yellows, with increased soil or air temperature to 28 degrees C. It would seem that both host and pathogen might share in this measurable response in their relationship. Andrus and Wade (3) have secured a significant difference in resistance of a cross between Corbett Refugee and Geneva Red Kidney bean to anthracnose in the greenhouse and in the field. In fact a segregating F2 gave a ratio of 14: 2 in the greenhouse and 15: 1 in the field at Beltsville while a 15: 1 ratio was obtained with a comparable F3 population in the greenhouse at Charleston. They conclude that "environment is an especially important factor in anthracnose reaction where certain genotypes are involved", and further "It would seem to be self evident in the field of plant breeding that any vegetable as highly developed and interbred as, for example, tomatoes, melons, peas and beans would possess extreme inter-varietal heterogeneity in respect to genotypes . .

For plant breeders faced with problems of adaptability, as many of us in the South and West are, a good deal of the work in experimental taxonomy has a direct hearing on our problems. It makes no real difference whether the features that fit a plant for a particular environment are inherited under other conditions or not. Whether the valuable characteristic represents an "environmental variation" from what may he considered the normal type elsewhere, or represents a new genetic combination that can maintain its individuality under other conditions makes a difference only if the climatic conditions the plant breeder faces are so variable that the usefulness of the so-called "environmental variation" is nullified. One of the difficulties in discussions of this kind is the almost universal human error of assuming that the familiar is the only true norm and that all other forms are "off-type". This suggestion obviously has its limitations, as the normal tends to be the modal type, and environmental variation as observed in the laboratory or under field conditions tends to be continuous. In the widest sense, distinct forms resulting from the interaction of a genetic complex with contrasting environments, do have genetic meaning both for the taxonomist and for the plant breeder.

The problem of adaptation among both wild. and cultivated plants is a matter of finding genes whose expression under a particular set of conditions favor growth and maintenance. The essential differences in the two cases lie in the numbers of individuals involved, in the methods of selection, and to no small extent in the economic motivation of the breeder. A better understanding of the processes of nature should be of value to the latter.

An important method in the experimental study of taxonomy is the transplantation of clonal divisions of different forms to a common environment or to several types of environments and noting differences in behavior. These have been of several kinds. Turesson (77) moved geographic races of a good many European species to a single garden. Some geographic varieties remain distinct when grown together, in other cases they are indistinguishable, showing that geographic types may or may not be the result of differences of genetic expression under different environments. Turesson stresses parallel varieties among geographic races of unrelated species when growing under the same climatic influences. He finds that a widely distributed species may have a special alpine type in one group of mountains but not in another where it also grows. He suggests that this may result from the loss of genes producing the alpine type from the species in one area but not in another. Thus occurrence of geographic races may be due either to environmental influence on gene expression or to the natural selection of genetic segregates or complexes that have special adaptive value. Clansen, Keck and Hiesey (16) list a number of plant characters unaffected by a change of climate. These include habit of branching, density of inflorescence, leaf shape, venation and texture, the character and density of pubescence, anthocyanin in the stems, flower size, shape and color, and seed-size and color. Where differences were observed, "Nearly every morphological character was found to depend upon a small series of genes, each of minor but cumulative effect." Traits having adaptive value such as time of flowering, frost resistance etc. also have a genetic basis.

Vavilov (80, 81, 82) has developed the idea of parallel variation among geographic races and species, calling this the "law of homologous series in variation". He suggests that where plant breeders observe adaptive characteristics in species related to the material they are working with, there might be a reasonable chance of finding or developing the character from their own or the more closely related wild material. The crossing of distinct geographic races may make it possible to transcend the limit of ordinary types.

Turesson (78) refers to the work of Massart (45), which I have not had an opportunity to consult, as an example of very marked response of a single genotype to different environments. Polygonum amphibium readily adapts itself as a land plant, a water plant or a dune type merely by growing divisions of a single individual under these distinct conditions. Each one would be considered a definite geographic race, which is the normal type for each environment.

In discussing the origin of genes responsible for well adapted climatic races Goldschniidt (29) credits Davenport and Cuénot with the suggestion that genes useful in a new environment arise by mutation and may be carried along by chance until they have an opportunity to express themselves and contribute to the survival of the species under the new conditions. This has been called preadaptation. He even goes so far as to say ". . . we must regard such preadaptational mutations as a prerequisite for the spreading of a species into new areas with different conditions, which would be inaccessible to the original form . . ." White (85) discusses the possibility of the existence of genes for cold hardiness among tropical species and those having a southern range. He cites the case of a native Texas pecan that was found to be fully hardy in Canada. Three species of Iris native to Texas proved to be hardy in New York. Occasional mutations for hardiness in tropical plants are likely to be lost if there is no change of climate to give them selective value.

Students of these problems realize that not just one gene may be involved in the genetic-environmental situation that determines the appearance and behavior of plants and animals, but whole groups of genes. The effective number of genotypes with which a worker can deal will depend upon the proportion of genes and gene combinations in any population that give a distinctive response to different environments. Bruman (12) has discussed this situation with respect to plant introduction. He observes that a change of climate may provide an opportunity for the expression of genetic factors that were entirely recessive in the old environment. Such changes may be of advantage or disadvantage either to the plant or to the breeder. He points out that the Navel orange is better in California than in either Florida or in its homeland, Brazil. It might be added that the pink-blush grapefruit appears to develop better color in Texas than in Florida. It is likely that most of the bud mutations of this type have been brought to light in Texas on this account. There is no reason to suppose that the rate of mutation is any higher in Texas than in Florida, although the larger number of Marsh trees in Texas may help to explain the situation.

There may be a lesson for the plant breeder in Fisher's theory of the origin of dominance (21, 22). He supposes that most mutations originally have some effect in the heterozygous condition, that is they are partially dominant to their wild type allel. As this effect is unlikely to be beneficial to the organism, any combination of genetic factors tending to cover up the effect of the new gene will have survival value and eventually this will become the normal wild type with the new gene fully recessive. Such a complementary effect might be made use of by the plant breeder in outcrosses of valuable but not fully adapted material to secure new genetic combinations that favor the development of the desired characteristic under a particular set of environmental conditions. The value of such a method will depend entirely upon the material available. It should be remembered that the breeding situation among crop plants is different from that among wild species both in the matter of the effective number of breeding individuals and in the basis of selection. The opportunity for crosses with wild types in many instances permits the incorporation of such recessive genes in the plant breeder's stocks. For example it seems likely that certain useful genes for fruit color and perhaps other characters may have been introduced incidentally in transferring factors for wilt resistance from Lycopersicon pimpinellifolium to L. esculentum (8, 61). It has even been suggested (4) that ". . . mutations which are pathological in one gene-complex may be harmless or even advantageous in another, and such effects are open to the influence of selection."

Dobzhansky (18) states that the survival of recessive genes in wild species depends a good deal on the size of the effective breeding population. The smaller the population, the less chance there is for the survival of genes having no selection value. Since the effective breeding population in most plant breeding work is extremely limited, we infer that hereditary factors of especial value for a particular area are frequently lost when the actual breeding and selection are done outside of that area.

Babcock (5) raised seedlings of wild tarweeds in the garden and noted wide variation the first year affecting plant size and habit, flower structure and color, size and pubescence of leaves, and other characters. Additional types were secured through subsequent selection and inbreeding. He points out that many of these variations are "types of abnormality common among species long domesticated". Hill (36) recounts a similar experience during the domestication of Primula malacoides. During its. first 4 years in cultivation varieties were established that involved an increased size of corolla, a flower color range from white to "deep mauve", double flowers, corolla shape, scent of the foliage and vegetative vigor. These variations parallel those of other species of Primula long in cultivation.

Some of the most important characteristics the plant breeder interested in adaptability has to deal with may he classed as physiological. Nilsson-Ehle (53) found that an apparently uniform variety of wheat would become more resistant to cold through the natural elimination of those individuals with genetic factors for tenderness. McKinney and Sando (48) have crossed spring wheat, which requires long warm days with winter wheat, which first needs cool short days. In order to classify the segregation in the F2 they found it necessary to grow populations both in the spring and in the fall. In discussing the theoretical basis of vernalization McKinncy (47) emphasizes that physiological response is conditioned by the genetic constitution of the plant and that ". . . dominance and segregation ratios in many characters must be considered within well defined environmental limits . . .”

Heyne and Laude (34) have tested the resistance of inbred lines of corn to high temperature in the laboratory, securing differential response that checks with field experience. They conclude that "the testing of seedlings for heat resistance can be relied upon with considerable assurance for distinguishing genetic differences in the drought tolerance of larger plants of different strains of maize. Hawthorn (33) has been able to select lines of Bermuda onions less apt to split and double. The point of chief interest to us in connection with this work on wheat, corn and onions is that varieties that are to all appearances entirely uniform, do carry valuable hereditary factors that can express themselves only under suitable environmental conditions. As far back as 1911 Hagedoorn (31) advised that in crosses to secure drought resistance, tests for resistance should be restricted to the F2 as any recessive factors might be lost in a susceptible F1. The suggestion applies with equal force to crosses for disease resistance due to recessive factors.

Considerable information is available in regard to general responses of crop plants to changes of the environment. For example, Flory and Walker (24) found that the cabbage head tends to be longer in proportion to width under southern as compared to northern conditions. Darrow and Waldo (17) report that the Missionary strawberry is earlier than Klondike in the South with relatively long winter days but is later in the North with short winter days. The genotype conditioning the response of the strawberry to day-length can only be determined by growing selections under different environments.

The effect of environment on the behavior of corn has received considerable attention. One of the early studies was made by Ness (51) in 1898 who grew several varieties of field corn at Ithaca, New York and at College Station, Texas. When grown at College Station most varieties were slightly later, shorter, had more suckers, longer, thicker ears, and a larger number of ears. Jones and Huntington (40) suggest that "the yield per acre is highest near the northward limit of possible cultivation". This may be partly due to adaptability, partly to the elimination of some of the common pest of corn by the cold winters. Mangelsdorf (44) reports that some varieties as Surcropper and Ferguson Yellow Dent have wide regional adaptability in Texas while others have very narrow limits for high production. Similarly some varieties as Mexican June have wide seasonal adaptability. Pearl and Surface (56) ascribe the regional adaptability of varieties of corn to the hereditary factors that have accumulated in any one variety. Credit for developing races of corn having genetic factors permitting the adaptability of races to widely different environmental conditions is given by Jenkins (38) to the American Indian.

Before drawing the moral for plant breeders inevitable to such a discussion as this let us review briefly some of the points that have been made: (a) single mendelian factors that have been studied genetically have been found to vary widely in their expression because of differences in environmental conditions; (b) under one set of conditions it may be impossible to distinguish between distinct genetic types while under other conditions they may he quite different in appearance; (c) factors of the environment that are responsible for these differences include moisture, temperature, light, nutrition and many geographic and cultural conditions that affect these things; (d) there must be appropriate environmental conditions before any gene or combination of genes can have selective value, either natural or in plant breeding, otherwise they may be entirely lost; (e) in tests, suitable conditions may have to be provided artificially; (f) the cumulative effect of modifying factors under a particular set of environmental conditions can be taken advantage of by the plant breeder in improving the adaptability of selections having special market appeal; (g) the value of any heritable character under a particular set of conditions bears no relation to its development or lack of development under other environmental conditions; (h) work in experimental taxonomy encourages the belief that the adaptability of many crops for southern and southwestern conditions can be materially improved by breeding and selection even though they have been developed primarily for other regions with quite different conditions; (i) genes of value in one area may be lost by breeding elsewhere, improvement might be expected in some cases through intervarietal crosses by accumulating genes from different varieties that may have a favorable effect directly or in combination; and finally (k) in other cases more rapid progress may be expected by outcrossing to wild forms where these are available or by making wide crosses among cultivated forms. Perhaps this summary carries its own moral.

As a matter of fact much of the breeding work in the South and Southwest has been and still is in line with these considerations. The United States Regional Vegetable Breeding Laboratory at Charleston, South Carolina, sends out segregating strains of vegetables to permit selection of the most favorable genetic combinations for each local condition, which does indeed vary widely in the region served by this laboratory. The Alabama Station has for years collected varieties of vegetables that have attained special local adaptability through selection for perhaps several generations in home gardens. Miller's selections of strains of vegetables especially adapted to conditions in Louisiana and Potter's selections of productive tung oil trees follow these principles. The program at the United States Horticultural Field Station at Cheyenne, Wyoming includes the collection of strains of horticultural plants adapted to the Southwest, many of them grown and perhaps developed by the Indians of that vast area. T. V. Munson was highly successful in crossing commercial varieties with selections of the native grapes of Texas and many of the standard varieties now grown were produced in this way by him. The work of Ness (52) in crossing the wild dewberry with the cultivated raspberry, and of Drain with raspberries for the South are also of this type. The most successful varieties of plums in Texas represent crosses between selections of native species and Japanese and European varieties. The current breeding program with fruits and vegetables at the Texas Station is based on these considerations.

The renewal of interest in breeding for increased adaptability to southern conditions evident in the past 10 years is very encouraging. As the work progresses we may expect an even larger accumulation of hereditary factors favoring quality and production under our conditions. This will make it increasingly easy to synthesize a variety according to certain specifications. There is still a good deal of spade work to be done. This means that we must discover new genes, judging their value to us not by their expression under a different environment, but by what they can do under conditions peculiar to our own locality, both as individual hereditary factors and in new combinations. With these it seems reasonable to expect that we can provide the plant material basis for an increasingly prosperous southern horticulture.


  1. ALLARD, H. A., and GARNER, W. W. Responses of some plants to equal and unequal ratios of light and darkness in cycles ranging from 1 hour to 72 hours. Jour. Agr. Res. 63: 305-330. 1941.
  2. ALLEN, C. E. Influences determining the appearance of sexual characters. Proc. Inter. Cong. Plant Sci. (4th) Ithaca 1: 333-343. 1929.
  3. ANDRUS, C. F., and WADE, B. L. The factorial interpretation of anthracnose resistance in beans. U.S.D.A. Tech. Bul. No. 810. 1942.
  4. ANONYMOUS. Genetics and ecology in relation to selection. Nature 138: 748-749. 1936.
  5. BABCOCK, E. B. Remarkable variations in tar weeds; many abnormalities found in plants long domesticated appear in first generation raised in garden. Jour. Hered. 15: 132-144. 1924.
  6. BARNES, W. C. Effects of some environmental factors on growth and color of carrots. N. Y. (Cornell) Agr. Exp. Sta. Mem. 186. 1936.
  7. BEADLE, G. W., TATUM, E. L., and CLANCY, C. W. Food level in relation to rate of development and eye pigmentation in Drosophila melanogaster. Biol. Bul. 75: 447-462. 1938.
  8. BOHN, G. W., and TUCKER, C. M. Studies on Fusarium wilt of the tomato. I. Immunity in Lycopersicum pimpinellifolium Mill. And its inheritance in hybrids. Mo. Agr. Exp. Sta. Res. But. 311. 1940.
  9. BRAINERD, E., and PETERSEN, A. K. Blackberries of New England — Their classification. Vt. Agr. Exp. Sta. Bul. 217. 1920.
  10. BRAUN, W. Opposite effect of environmental factors on similar phenotypes. Amer. Nat. 72: 189-192. 1938.
  11. BRESLAVEC, L. Researches on development of the flower in hemp whose sex has been changed under 'the influence of photoperiodism. Genetica 19: 393-412. 1937.
  12. BRUMAN, A. J. Genetic aspects of plant introduction. An approach to the heredity-environment problem in plants. Sci. Monthly 46: 120-131. 1938.
  13. BUXTON, B. H., and DARBISIIIRE, F. V. On the behaviour of "anthocyanins" at varying hydrogen-ion concentrations. Jour. Genetics 21: 71-79. 1929.
  14. CHILD, G. Phenogenetic studies on scute-1 of Drosophila melanogaster. I. Genetics 20: 109-126, 127-155. 1935.
  15. —— Phenogenetic studies in scute of D. melanogaster. III. The effect of temperature in scute 5. Genetics 21: 808-816. 1936.
  16. CLAUSEN, J., KECK, DAVID, D., and HIESEY, W. M. Regional differentiation in plant species. Biological Symposia 4: 261-280. Lancaster. 1941.
  17. DARROW, G. M., and WALDO, G. F. The practical significance of increasing the daily light period of winter for strawberry breeding. Science 69: 496-497. 1929.
  18. DOBZHANSKY, TH. Genetics and the Origin of Species. Columbia Univ. Press. New York. 1937.
  19. EKER, R. Further studies on the effect of temperature on the manifestation of the short-wing gene in Drosophila melanogaster. Jour. Genetics 38: 201-227. 1939.
  20. EMERSON, R. A. Control of flowering in teosinte. Short-day treatment brings early flowers. Jour. Hered. 15: 41-48. 1924.
  21. FISHER, R. A. The evolution of dominance. Biol. Revs. 6: 345-368. 1931.
  22. —— The evolutionary modification of genetic phenomena. Proc. 6th Inter. Cong. Genetics 1: 165-172. 1932.
  23. FLANDERS, S. E. The temperature relationships of Trichogramma minutum as a basis for racial segregation. Hilgardia 5: 395-406. 1931.
  24. FLORY, W. S., JR., and WALKER, J. C. Effect of different environments on head shape in Marion Market cabbage. Proc. Amer. Soc. Hort. Sci. 37: 778-782. 1940.
  25. FORD, E. B. The theory of dominance, Amer. Nat. 64: 560-566. 1930.
  26. GARNER, W. W., and ALLARD, H. A. Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants. Jour. Agr. Res. 18: 353-606. 1920.
  27. —— Duration of the flowerless condition of some plants in response to unfavorable lengths of day. Jour. Apr. Res. 43: 439-443. 1931.
  28. GOLDSCHMIDT, RICHARD. The gene. Quar. Rev. Biol. 3: 307-324. 1928.
  29. —— Some aspects of evolution. Science 78: 539-547. 1933.
  30. —— Physiological Genetics. McGraw-Hill Co., New York. 1938.
  31. HAGEDOORN, A. L. Facteurs genetiques et facteurs du milieu dans l'aneliolation et l'obtention des races. Conf. Intern. Genetique. Paris. 1911: 132-135. 1911.
  32. HARNLY, M. H. The temperature-effective periods and the growth curves for length and area of the vestigial wings of Drosophila melanogaster. Genetics 21: 84-103. 1936.
  33. HAWTHORN, L. R. Behavior of certain characters in breeding yellow Bermuda onions. Proc. Amer. Soc. Hort. Sci. 36: 668-673. 1939.
  34. HEYNE, E. G., and LAUDE, H. H. Resistance of corn seedlings to high temperatures in laboratory tests. Jour. Amer. Soc. Agron. 32: 116-126. 1940.
  35. HESTON, W. E. Bent-nose in the Norway rat. A study of interaction of genes and diet in the development of the character. Jour. Hered. 29: 437-448. 1938.
  36. HILL, A. W. The history of Primula malacoides, Franchet, under cultivation. Jour. Genetics 7: 193-198. 1918.
  37. HONING, J. A. Canna crosses. VII. Two types of Canna glauca with anthocyanin in the labellum, one dominant, the other recessive to pure yellow, Pisum-type or zea type a question of temperature. Genetics 21: 325-344. 1939.
  38. JENKINS, M. T. Influence of climate and weather on growth of corn. U.S.D.A. Yearbook 1941: 308-320. 1941.
  39. JENNINGS, H. S. Heredity and environment. Scientific Monthly 19: 225-238. 1924.
  40. JONES, D. F., and HUNTINGTON, E. The adaptation of corn to climate. Jour. Amer. Soc. Agron. 27: 261-270. 1935.
  41. KNOTT, J. E. The effect of temperature on the photoperiodic response of spinach. N. Y. (Cornell) Agr. Exp. Sta. Mem. 218. 1939.
  42. MAGRUDER, Roy, and ALLARD, H. A. Bulb formation in some American and European varieties of onions as affected by length of day. Jour. Agr. Res. 54: 719-752. 1937.
  43. MAGRUDER, R., BOSWELL, V. R., EMSWELLER, S. L., MILLER, J. C., HUTCHINS, A. E., WOOD, J. F., PARKER, M. M., and ZIMMERLEY. H. H. Descriptions of types of principal American varieties of orange-fleshed carrots. U.S.D.A. Misc. Pub. 361. 1940.
  44. MANGELSDORF, P. C. Corn varieties in Texas; their regional and seasonal adaptation. Tex. Agr. Exp. Sta. But. 397. 1929.
  45. MASSART, J. L'accomodation individuelle chez le Polygonum amphibium. Bul. d. jardin Bot. de L'Etat d Bruxelles 1: 1902. (Reported by Turesson Hereditas 3: 211. 1922).
  46. McCLELLAND, T. B. Studies of the photoperiodism of some economic plants. Jour. Agr. Res. 37: 603-628. 1928.
  47. McKINNEY, H. H. Vernalization and the growth-phase concept. Bot. Rev. 6: 25-47. 1940.
  48. —— and SANDO, W. S. Earliness and seasonal growth habit in wheat as influenced by temperature and photoperiodism. Jour. Hered. 24: 169-179. 1933.
  49. McPHEE, H. C. The genetics of sex in hemp. Jour. Agr. Res. 31: 935-943. 1925.
  50. MORGAN, T. H. The role of the environment in the realization of a sex-linked mendialian character in Drosophila. Amer. Nat. 49: 385-429. 1915.
  51. NESS, H. Variation in Indian corn when brought from New York to Texas. Trans. Texas Acad. Sci. 1898: 73-78. 1899.
  52. —— Breeding work with blackberries and raspberries. Jour. Hered. 12: 449-455. 1921.
  53. NILSSON-EHLE, H. Mendelisme et acclimatation. (On acclimatization by recombination of mendelian factors.) Conf. Intern. Genetique 4 Paris 1911. Compt. Rend. 136-157. 1913.
  54. OWEN, F. V. Hereditary and environmental factors that produce mottling in soy beans. Jour. Agr. Re: . 34: 559-587. 1927.
  55. —— BURGESS, I. M., and BURNHAM, C. R. The influence of environmental factors on pigment patterns in varieties of common beans. Jour. Agr. Res. 37: 435-.442. 1928.
  56. PEARL, R., and SURFACE, F. M. Growth and variation in maize. Nat. Acad. Sci. Proc. 1: 222-226. 1915.
  57. PETERSON, A. A biological study of Trichogramma minutum Riley as an egg parasite of the oriental fruit moth. U. S. D A. Tech. Bul. 215. 1930.
  58. PLATENIUS, HANS, and KNOTT, J. E. Studies in onion pungency. Jour. Agr. Res. 62: 371-379. 1941.
  59. PLUNKETT, C. R. The interaction of genetic and environmental factors in development. Jour. Exp. Zool. 46: 181-244. 1926.
  60. POEHLMAN, J. M. Study of the relative adaptation of certain varieties of soybeans. Mo. Agr. Exp. Sta. Res. But. 255. 1937.
  61. PORTE, W. S. Development of disease resistant varieties of tomatoes. Rep. Md. Agr. Soc. and Md. Farm Bur. Fed. 20: 264-269. 1936.
  62. RATSEK, J. C. Some factors causing fading in color of rose blooms. Proc. Amer. Soc. Hort. Sci. 39: 419-422. 1941.
  63. RICHEY, F. D., and SPRAGUE, G. F. Some factors affecting the reversal of sex expression in the tassels of maize. Amer. Nat. 66: 433-443. 1932.
  64. ROBERTS, R. H., and STRUCKMEYER, B. E. The effect of temperature upon the responses of plants to photoperiod. Science 85: 290-291. 1937.
  65. —— Photoperiod, temperature, and some hereditary responses of plants. Jour. Hered. 29: 94-98. 1938.
  66. —— Further studies of the effects of temperature and other environmental factors upon the photoperiodic responses of plants. Jour. Agr. Res. 59: 699-709. 1939.
  67. —— The effect of environment upon the variability within a population of plants. Proc. Amer. Soc. Hort. Sci. 37: 267-268. 1940.
  68. SAX, K. A genetical interpretation of ecological adaptation. Bot. Gaz. 82: 223-227. 1926.
  69. SCHAFFNER, J. H. Sex reversal and the experimental production of neutral tassels in Zea mays. Bot. Gaz. 90: 279-298. 1930.
  70. —— The fluctuation curve of sex reversal in staminate hemp plants induced by photoperiodicity. Amer. Jour. Bat. 18: 424-430. 1931.
  71. STRUCKMEYER, B, E., and ROBERTS. R. H. The effect of photoperiod and temperature upon the growth of seedlings and cuttings. Amer. Jour. Bot. 26: 694-697. 1939.
  72. SURRARRER, T. C. The effect of temperature on a mottled-eye stock of Drosophila melanogaster. Genetics 20: 357-362. 1935.
  73. THOMPSON, H. C. Temperature as a factor affecting flowering of plants. Proc. Amer. Soc. Hort. Sci. 30: 440-446. 1934.
  74. —— Temperature in relation to vegetative and reproductive development in plants. Proc. Amer. Soc. Hort. Sci. 37: 672-679. 1940.
  75. —— and KNOTT, J. E. The effect of temperature and photoperiod on the growth of lettuce. Proc. Amer. Soc. Hort. Sci. 30: 507-509. 1934.
  76. THOMPSON, ROSS C. Genetic relations of some color factors in lettuce. U. S. D. A. Tech. Bul. 620. 1938.
  77. TURESSON, G. V. The species and the variety as ecological units. Hereditas 3: 100-113. 1922.
  78. —— The plant species in relation to habitat and climate. Contributions to the knowledge of genecological units. Hereditas 6: 147-236. 1925.
  79. —— The selective effect of climate upon the plant species. Hereditas 14: 99-152. 1930.
  80. VAVILOV, N. I. The law of homologous series in variation. Jour. Genetics 12: 47-89. 1922.
  81. —— The process of evolution in cultivated plants. Proc. Intern. Congr. Genetics, 6th, 1932, 1: 331-342. 1933,
  82. —— The new systematics of cultivated plants. In The New Systematics. Julian Huxley Ed. Oxford. 1940.
  83. VOGELE, A. C. Effect of environmental factors upon the color of the tomato and the watermelon. Plant Physiol. 12: 929-955. 1937.
  84. WALKER, J. C., and SMITH, ROSE. Effect of environmental factors upon the resistance of cabbage to yellows. Jour. Agr. Res. 41: 1-15. 1930.
  85. WHITE, O. E. Mutation, adaptation to temperature differences and geographical distribution in plants. Inter. Kong. Vererb. 5 Berlin 1927 Verhandl. 2: 1575-1586. 1928.
  86. ZELENY, C. The temperature coefficient of a heterozygote. Biol. Bul. 44: 105-112. 1923.