The forms of life of each natural geographical region (vegetation, animals, and microorganisms) are primarily characteristic of the particular region or type of region. They are closely interrelated in an unstable equilibrium with each other and with climate, soil materials, physiography and time. Such a description of the natural environment of man is the product of twentieth century science crystallized out of scattered nineteenth century beginnings.
Livingston (1921) designated three phases in the evolution of this twentieth century point of view on vegetation, and in some degree a similar framework may be applied to the other forms of life. Early nineteenth century biological leaders, from Alexander von Humboldt to Adolph Engler, were interested primarily in the distribution of the forms of life over the earth's surface and in the composition of the different populations, a process of description and classification. The second phase began with Darwin's Origin of Species (1859). In biology Darwin's emphasis on the idea of dynamics meant reinterpretations of accumulated knowledge, among which was the relation of structure to the conditions of environment, but with the emphasis on structure to the neglect of environment. The third phase was the study of "natural assemblages of plants"- vegetation - and of animals. This was a combination of pre-Darwinian study of distribution and post-Darwinian study of relations with environment, and to this is given the name ecology. Asa Gray (l859, 1872, 1878), in America, was credited with inspiring Adolph Engler to lead the study of vegetational history and development, migrations and survivals (Gleason, 1922). Grisebach (1838) is credited with formulating the concept of plant communities; Holts (1885-l887) with first recognizing fully the importance of the development of vegetation by succession of stages to a climax formation. The decade of the l890s produced an outstanding triumvirate of ecologists, Warming, Drude, and Schimper. J. E. B. Warming (1891) was credited with giving the first consistent account of succession on sand dunes and in his major work in 1895, Plantesanfund, rewritten and translated from the Danish into English (1909) under the title of Ecology of Plants, an introduction to the study of plant communities. Warming's book presented the new point of view in so comprehensive a manner as to elicit the comment (Cowles, l909) that it was "the most important ecological work in any language." Because of influence on Roscoe Pound and F. E. Clements, their estimate (1897) of Oscar Drude's work Deutschlands Pflanzengeogrophie (1896) is significant to this narrative- his "concept of plant formation is undoubtedly the correct one," his book a classic, and his conclusions final for the present. The third landmark in the new approach was A. F. W. Schimper's Pflanzengeogrophie auf physiologischer grundlage (1898) translated into English in 1903 as Plant geography on a physiological basis.
While Europe was engaged along these lines of scientific innovation, Americans were devoting their attention primarily to the older type of activities of describing and cataloguing the modern distribution of forms of life. In defense of them it is said (Gleason, 1922) that this was a natural and proper objective, because in a biological sense the American continent was largely unexplored. They were making an inventory of a new country. Only upon such foundations could the other types of scientific investigation be built. In 1889, C. H. Merriam began his experimental biological survey work on the San Francisco mountain, later applied to other regions. In his own account (1894) of this new departure he claimed,
One result of this first survey was the complete overthrow of the principal faunal areas previously recognized in the United States, and a radical change in our conception of the principles involved. In ascending the mountain a succession of climatic belts was traversed, similar to those encountered in journeying northward from the Southern States to the polar sea, and each belt was found to be inhabited by a distinctive set of animals and plants.
The next step was to generalize from the zones found on the mountain and to apply this zone system to the geographical distribution of life on the North American continent. Merriam's own summary reported that,
The principles of geographic distribution of terrestrial animals and plants in the Northern Hemisphere were clearly recognized in 1889, the correlation of the life zones was completed in 1892; the laws of temperature control were formulated in 1894. The work remaining to be done relates to details.... It appears, therefore, that in its broader aspects the study of the geographic distribution of life in the Northern Hemisphere is completed. The primary regions and their principle subdivisions have been defined and mapped, the problems involved in the control of distribution have been solved, and the laws themselves have been formulated.
There is no question that the contribution made was revolutionary so far as it went, but it was limited in its outlook to modern distribution, it was a single factor hypothesis, and it was static. The new point of view being developed in Europe was concerned not only with modern distribution, but also with a question, of more significance scientifically, of how and why modern distribution came about and its relation to contemporary problems. According to Merriam's static point of view he had finished his task, except for some details; but according to the new dynamic concepts, he was just ready to begin the investigation which should explain how and why, and test the validity of his life-zone hypothesis.
At the turn of the century the time had arrived in America for a new orientation and a synthesis on the basis of the accumulation of scientific materials and ideas. There was little that could be truly new in the process and many minds were engaged. It is difficult to know how to apportion credit for specific developments. All were building on the work of the late nineteenth century Europeans, particularly of Engler, Warming, Drude, and Schimper. C. C. Adams (b. 1873, Ill.,) was most directly in the line of development from the Asa Gray influence on historical distribution and development. He was applying (1902) the dynamic approach of the European school and utilized the findings of geology and paleontology. He found two centers of refuge for fauna and flora of North America during the glacial invasions, the southeast and the southwest, and from them came the redistribution northward over the continent during post-glacial times.
In the investigations of vegetational processes of recent times two centers of study were conspicuous, the Universities of Chicago and Nebraska. At the former institution under John Merle Coulter a number of men received all or part of their training: H. C. Cowles (b. 1869, Conn.), B. E. Livingston (b. 1875, Mich.), and in part E. N. Transeau (b. 1875, Pa.), who finished his work at the University of Michigan. In Nebraska, of the students of C. E. Bessey, E. F. Clements became the outstanding leader, and H. L. Shantz received his training there. These two great teachers, Coulter and Bessey, have been placed for the first time in their proper historical perspective in Rodger's biography, John Merle Coulter (1944). A considerable part of the work of this group of Middle West Americans dealt with problems relating to the forest and grassland along the zone of transition or with the grassland itself. They were young men who were contributing to the creation of a new field of science. When publishing their first major contributions Adams was 29, Cowles was 30, Clements was 24, Livingston was 29, and Transeau was 28 years of age.
H. C. Cowles had begun his training as a geologist under Chamberlin and Salisbury at the University of Chicago and later turned to botany under Coulter. The result was a combination for the first time, in his study of Lake Michigan sand dunes (1899), of physiography with botany, according to the dynamic view of vegetation. Two years later in "The physiographic ecology of Chicago and vicinity" Cowles is said to have given the concepts of succession and climax their "first adequate expression." To him the climax was not permanent:
The condition of the equilibrium is never reached, and when we say that there is an approach to the mesophytic forest, we speak only roughly and approximately. As a matter of fact we have a variable approaching a variable rather than a constant.
This cautious avoidance of anything like a rigid closed system was said to be an important characteristic of his influence (Cooper, 1935). Clement's first major work (1898, 1900) was done jointly with Roscoe Pound as senior author, but his ideas were not fully formed. The quadrat method of quantitative study of plant communities was first devised for this study and became a standard procedure for certain types of quantitative investigation. In 1904 Clements "made the first attempt to organize the whole field of present-day succession, and to connect the structure of vegetation with its development...." Here appeared for the first time his organistic idea of plant communities: "The concept was advanced that vegetation is an entity, whose changes and structures are in accord with certain basic principles in much the same fashion that the functions and structures of plants follow definite laws." The next year he stated it again: "The formation was regarded as a complex organism, possessing functions and structures, and passing through a cycle of development similar to that of a plant" (Clements, 1916). In 1916 Clements published what was said to be "the first systematic monograph of the phenomena of succession...." Three important criticisms were directed at this notable work, the exclusive climate theory of vegetation, a new terminology that was "too complete:" and his use of an extreme form of the concept of organism in dealing with plant communities (Tansley, 1916). "The first quantitative study of the reactions and successions of a great grassland vegetation" was given by Shantz in 1911 (Clements, 1916). The study by Transeau, of 1905, correlating climate centers with vegetation centers, illustrates the process of synthesis of the contributions of others as the basis for a new and significant step in the enlargement of scientific knowledge. Referring to Adams's studies of "centers of distribution", as well as his own, he pointed out four such forest centers in eastern North America; the Northern conifer forests of the Laurentian plain, the deciduous forest centers of the Middle Atlantic, the Southern conifer forest center of the Atlantic Gulf States, and the Insular Tropical climax formation of Southern Florida centering in the West Indies. All but the last possessed a common grassland border where the plant societies of the great plains met these forest societies.
Referring next to the work of Cowles, Transeau emphasized the role of physiography in contributing to the plant successions. Part of the problems of analyzing vegetation of eastern North America could be explained by physiography as Cowles had done in his studies, but part of it could be explained only on the basis of the meeting and intermingling of plant societies of two or more geographical centers. Thus the first three stages in plant succession of the southern Great Lakes region were attributed to the northern conifer forest associations influenced by physiographic factors, but the fourth, or climax stage, was determined by the invasion of the beech-maple forests from the central deciduous forest center. Without competition from other shade-making and shade-resistant deciduous trees that did not thrive that far north from the deciduous center, the beech-maple society became dominant over the northeastern conifers. The prairie peninsula of Illinois and Indiana was a projection of the grassland between the northeastern conifer and the deciduous forest centers, or more accurately, it had been the transition zone on the periphery of the three vegetation centers, and still was, except that the beech-maple forest had crossed over or around the grassland peninsula in its northward expansion and crowded out the earlier conifer succession to the northward of the prairie peninsula. Around the northern edge and to the south of the prairie peninsula only the oak-hickory combination of the deciduous forest center had occupied the western central periphery and around the south edge reached west as far as eastern Kansas. There the plains grasses met in the transition zone the shrub and still farther east the oak-hickory trees of the eastern deciduous forest center. On the Minnesota border and on the Texas border the plains grasses met different forest societies, the northeastern conifers and the southern conifers.
Transeau's major contribution was to formulate the next step in analysis; the correlation of climate centers with these vegetation centers. No single factor of climate was sufficient, so he hit upon the idea of precipitation-evaporation ratio as a single index for mapping the climate and vegetation regions. To determine this ratio, he divided the annual rainfall by the depth of evaporation in inches: "The depth of evaporation depends upon the temperature of the evaporation surface, the relative humidity of the air, and the velocity of the wind. These are the same climatic factors which most powerfully affect transpiration, and which must be of great importance in determining the geographic range of plants."
Rainfall data were available for substantial periods of time in the eastern United States, but the only evaporation data on record that could serve the purpose were those of T. Russell (1988) collected for a single year, July 1, 1887 to June 30, 1888. Transeau realized that not all the water that falls is available to plants, and that evaporation data from a water surface even if adequate for a period of years were not an accurate measure of water given off by plants, but nevertheless "the figures have a comparative value in both cases and when combined probably give a fairly correct idea of the distribution of these climatic factors in the eastern United States." Transeau does not appear to have appreciated how abnormal a climatic year of heat and drouth was represented in the Russell data for 1887-1888, and it is probably just as well, or he might have been discouraged in using it at all. As will come out later in this discussion, possibly this abnormality was after all by coincidence a factor of safety, according to the hypothesis of R. J. Russell relative to the ecological significance of climatic extremes. Transeau plotted his ratio values on a map of the eastern United States which he explained as follows:
It will be noted that the Great Plains are marked by a rainfall equal to from 20 to 60 percent of the evaporation called for. The prairie region where the forests are confined to low grounds is identical by a ratio of from 60 to 80 percent. Its limits as indicated show a remarkable agreement with the actual distribution of the prairie. The region indicated by ratios between 80 and 100 percent is more or less coincident with the occurrence of 'oak openings', 'open forests', and 'groves' on the uplands, and dense forests on the low lands.The southeastern area where the rainfall is from 100 to 110 percent of the evaporation, corresponds to the region of the deciduous forest center. The distribution of the ratios above 110 percent in the region of the coastal plain is remarkably similar to the position of the Southeastern Conifer forest center. In the southern Appalachians the ratio also rises above 110 percent and coincides with the occurrence of the southern extension of the northeastern forests. No data are available for the mountainous parts of Pennsylvania, so that this apparently isolated area may be climatically connected northward along the higher mountain crests. The Northeastern Conifer forest center is marked by ratios above 100 percent and centering in the St. Lawrence basin. It is probably that the northern limits of this formation will not be indicated by rainfall-evaporation ratios, for the factors commonly accepted (Schimper, 1903, p. 168) as determining the northern limits of forests are very different from those causing the boundaries of other formations. It should also be stated that since climates are constantly changing and effects may be behind their causes, no map of present climatic conditions can hope to do more than approximate the present distribution of plants. Geographic and historical relations must be constantly borne in mind.
The preceding studies deal with the larger phases of the relations between vegetation and environmental factors such as historical geology, physiography, migration, and succession of plant communities, and climate. The next phase in the development of ecology was derived from the twentieth century emphasis on physiology and the correlation of this with the others. The goal of this approach was to reduce ecology definitely to a quantitative and objective scientific basis with a view to making quantitative determinations of the role of the several factors involved. In 1905 the first compilation of relative humidity data was prepared, by W. B. Stockman, covering the period 1888-1901 for 130 stations. It made possible the study of relative humidity in relation to plants. Livingston became a leader in America in the work on evaporation in relation to plants, particularly because of his invention (1904) of the porous cup atmometer for measuring evaporation as a climate factor. This instrument and its later modifications was widely used in America and extensively in Europe where it displaced for many purposes the older Picke atmometer (1872). Livingston's work (1906, 1908) on desert plants inspired many others to study evaporation in relation to several aspects of plant communities and succession (Transeau, 1908; Fuller, 1911, 1912; Gleason and Gates, 1912). Livingston (1906) was a leader also in the study of transpiration, root systems, root physiology, and soil moisture, but the most important early study as related to the grassland was that of Briggs and Shantz (1911, 1912, 1913, 1916) on the water requirements of plants in the great plains. In a region of limited rainfall the root systems of plants were very large in proportion to the surface parts-in fact most of the plant was in the soil and as the significance of this relationship became more fully appreciated, the roots and the soil processes became more important as subjects of investigation.
The most comprehensive study of the distribution of vegetation in the United States, as related to climatic conditions, and the first of its kind for the United States, which employed on so extensive a scale the new physiological and quantitative techniques, was that of Livingston and Shreve in 1921. They emphasized that it was "to be regarded only as a beginning along a line that holds forth very great promise." In respect to the climatic factor they warned that methods and interpretation were "woefully inadequate." In this connection, they insisted that new methods were necessary for obtaining and interpreting climatic records. The ecologist was interested in physiological effects, not in meteorological causes.
A further limitation of the work was pointed out by the authors. Data were available primarily only for environmental conditions above the surface of the soil and consequently the book was limited to that scope. Little that was satisfactory had been done with roots and soil in relation to plant distribution. On the root question, Livingston had pointed out, in 1909, that the neglect was such "that we are at present more densely ignorant of these organs than of any other equivalent portion of the plant," and concluded that from such investigations as had been made recently there was evidently a need for a new hypothesis of soil fertility. Livingston has said in 1917
During the last quarter-century we seem, indeed, to have been mainly engaged in discovering new problems, rather than in solving the problems dealt with by earlier workers. In many cases the problem has been analyzed into several component problems, each one of the later appearing as difficult now as did the less thoroughly analyzed problem to the writers of a few decades ago.
As the years passed many advances were made in answering some of the questions and in further redefining the unanswered problems. As the characteristics of xenophytes, or so-called drouth-adapted plants, are a matter of special interest to the low-rainfall areas of the grasslands, a few of the leading contributions to plant physiology dealing with the relations of plants to water will be pointed out in this discussion. It is important to notice, among other things, that drouth-resistance as commonly used in agricultural discussions has a meaning quite different from its use in plant physiology. Originally proposed by Kearney and Shantz in 1911, the latter had settled, by 1927, upon a classification, which placed drouth-adapted plants in four groups: drouth-resisting, drouth-escaping, drouth-evading, and drouth-enduring. As used in such a classification, drouth-resisting plants were those that stored their water-supply within the plant itself either above or below ground, and during drouth were enabled to continue growth for long periods of time. Most conspicuous were the cacti, but some non-succulent plants belonged to this group such as African grassland trees cited by Shantz. The drouth-escaping plants include such examples as the desert ephemerals that grew during short favorable seasons, matured, and seeded quickly. The plant itself possessed no drouth-enduring characteristics, but depended for perpetuation upon seed which survived the drouth. The drouth-evading plants conserved moisture in one of several ways, by small areal parts, or by wide spacing, or by efficiency of transpiration, or by restriction of growth. Many of the cereals of dry-land agriculture belonged to this group. The divergences in the physiological characteristics of plants in this category were illustrated further by variations in leaf temperature. On a hot, bright day the leaf temperature of alfalfa might by several degrees below the atmospheric temperature while the leaf temperature of sorghum might be above atmospheric temperature. Alfalfa possessed a high transpiration rate and sorghum a low transpiration rate. The drouth-enduring plants made additional adjustments including thickened, leathery, or woody leaves. During drouth they made no growth, and with prolonged drouth might drop their leaves or part of them and even twigs might die, but when water was again available they revived quickly. Desert shrugs were most typical of this group. Maximov (1929) objected to the inclusion of cacti among drouth-resisting plants because they lacked the characteristic xerophytic feature of high osmotic pressure. It is evident from this classification that there was no single answer to the problem of the relation of plants to water. Each of the several groups met their water requirements in different ways and there was the widest difference among the members of each class.
Maximov (1929) summarized the results of approximately a generation of research workers on the water-relations of plants, along with his own conclusions. His book marked a major revolution in the field. In respect to the anatomical characteristics of xerophytes, he concluded that all cells were smaller, the networks of veins were denser because of the smaller cells, and about the same number in the spaces between veins, the cell walls were thickened, the number of stomata were greater, and special protective devices were present, such as a thickened cuticle to reduce intercellular water loss. In respect to physiological characteristics, Maximov (1929, 1931) rejected the traditional views of xerophytic adaptation which were based on the assumption of morphological changes to reduce intensity of transpiration, and emphasized instead the greater intensity of transpiration and assimilation, a higher osmotic pressure, and an increased capacity to endure wilting. He was convinced that the secret of drouth endurance lay in the protoplasm, which experienced a change analogous possibly to the hardening process associated with frost resistance. In order to correct some misunderstanding of his emphasis on greater intensity of transpiration (1929), he restated his position (1931), to emphasize that because of the several types of xerophytes the same characteristics were not common to all.
Experiments on osmotic values conducted at the Desert Laboratory of the Carnegie Institution represented an important field of investigation of the properties of protoplasm in relation to water. The general scope of the work can be followed through the annual reports of Shreve and Mallery, in the Yearbooks, (Particularly Yearbook No. 33, 1934). These studies emphasized that in creosote bush (Larrea) the osmotic values varied widely within the same day; that in general they reached their minimum at the end of the rainy season, their maximum at the end of the dry season; that the variation in values between plants was widest during drouth, individual plants responding differently to the progressive impact of extreme conditions; and that the greater the range of osmotic values, the more successful and uniform the growth of the creosote bush (Mallery, 1935).
In rejecting the assumption that the primary functions of the stomata were related to the control of water loss, attention was turned anew to the positive aspects of these structures. Among the criticisms directed at research in plant physiology, one was the neglect of the most important of all plant processes, photosynthesis. In this field the hypothesis of Scarth (1927, 1932) became generally accepted (Seifriz, 1938) that the stomatal movements in plants were controlled by the effect of light on bio-chemical processes tending to produce alkalinity of the cell protoplasm which increased osmotic pressure and opened the stomata, while conversely darkness reversed the process. These assumptions related stomatal movements, therefore, to the basic physiological processes of photosynthesis.
The older theory of the functions of the stomata as regulating transpiration, particularly, the assumption that the stomata closed to reduce or prevent water loss, was in direct conflict with the requirements of photosynthesis which required free exchange of gases for the conversion and assimilation of nutrients in the process of plant growth. The view held by Maximov and others resolved substantially this difficulty as applied to those xerophytes that possessed a high rate of transpiration. Maximov (1931) explained that the closing of the stomata as a defense against water loss did not occur until the wilting point was reached. The real test of drouth resistance was not the transpiration criterion, but the test that came with wilting; "The capacity to survive long periods of drought and dehydration of their tissues without injury, or with only slight injury." At this stage structural features such as thick cuticle did promote economy in the use of the water that remained in the plant, but the principal defense was in the protoplasm itself. Needless to say the views of the Maximov group aroused much adverse criticism, but they constituted a major landmark from which new advances were to be measured. One of the several significant criticisms was that of Wood (1934) based on Australian material, that xerophytism must be viewed on a broader basis than water relations alone. In a sense, however, this was not so much a disagreement as an advance, because he directed attention also to the problem of protoplasm.
In its transit around the globe, modern civilization was concerned mostly with the temperate zones where problems of light and temperature attracted less attention than water requirements of plants and animals. The extension of man's activities into the higher latitudes and into the tropics emphasized those neglected factors. Plant physiology gave to them a new significance in all environments. With respect to light, plants may be classified as long-day, short-day, intermediate, and day-neutral. Some plants require long and some require short seasons of a particular day-length and temperature to grow, to develop toward reproduction, and to mature seed. In the high latitudes the days are long and the growing season short. In the tropics, plants live under conditions of approximately equal periods of light and dark-short days-, and nearly continuous high temperatures throughout the year. Plants of the same genetic value may react quite differently to dissimilar environments, the factors of one region releasing characteristics inhibited in another. Geographical races or strains within the same species of native vegetation may show different responses to light and temperature, when moved from one latitude to another. A plant promising in one environment may be a disappointment in another, and the reverse (Whyte, 1946; Yarnall, 1942). Problems of distribution, migration, or adaptation, as applied to plants living in natural conditions or in domestication, require much rethinking when described in this framework of developmental physiology.
Plant physiologists distinguish between two aspects of the life of a plant, "growth" and "development." By growth is meant increase in vegetative size, and by development is meant the progress of the plant toward reproduction; flowering and seed formation. The German botanists, G. Klebs, and G. Gassner, were pioneers in the developmental approach, their more significant contributions being published first in 1918. Among Americans, the significant publications of H. A. Allard and W. W. Garner on light relations start in 1920. Those of the Russians, led by T. D. Lysenko and beginning about 1928, dealt with both light and temperature. Particularly significant was the broadening of range and volume of contributions in all research centers after about 1935. The most recent survey of the accumulated literature, which affords some perspective on its practical significance, is that of R. O. Whyte, Crop production and environment, issued late in 1946. Most of the following discussion is derived from Whyte's book as a basing point, being supplemented by other studies particularly pertinent to the North American Grassland.
Sharp differences are conspicuous in the theoretical aspects of developmental plant physiology. This is more or less characteristic of most new fields of investigation. The Russians, led by Lysenko, formulated the theory of phasic development; that growth and development follow definite phases which must proceed in rigid sequence and that one phase must be completed before the next can continue. Outside Russia, this view is largely questioned, or rejected outright. In nature, the factors that would initiate these phases can hardly influence a plant in the exact succession required by the Russian theory.
Vernalization is the name given to the method of modifying plant development so that the production of the seed may be advanced in time. A plant of winter habit may be made to produce a crop if planted in the spring, or a plant of spring habit may be made to mature earlier than otherwise. Klebs and Gassner applied low temperature treatment to seeds that would be the equivalent of the influence of wintering. Lysenko carried the procedure farther, applying his phasic development theory. Under the rigid sequence of phases, the Russians held that the process was irreversible. English experiments came to the opposite conclusion (Whyte, 1946). The system of developmental physiology raised the question of when the influence of temperature began, and answered that it might begin during the formation of the seed on the mother plant. This introduced the concept of physiological age as distinguished from actual age of the plant dated from the time the dormancy of the seed was actually broken. In practical application, the study of the growth and development of a plant must take into account the environment under which the seed was produced. This helps to explain the peculiarities of behavior of seeds introduced from different environments, and the contrasts in the behavior of introduced seeds with the seeds produced in the new environments.
The disagreements among scientists over vernalization are a reminder of the discussions among the farmers of central Kansas (1866, 1867) of the effect of wintering upon wheat varieties. It was generally agreed that winter wheat must undergo a wintering process if it was to produce a crop. Some argued that the seed should be planted even if conditions were not favorable for its growth, because the seed would thus be exposed to cold and the result would be equivalent to the winter effect upon the growing plant. They discussed also the latest date at which wheat could be planted, if it were to complete the desired cold-conditioning. One variety, Odessa, imported from Russia, was popular temporarily during the late 1870s partly because of the claim that it would make a crop either as a winter or as a spring wheat.
The subject of genetics, as related to developmental physiology, is particularly confused. The formal genetics derived from Mendel and Morgan, based inheritance upon characteristics associated with genes. The developmental genetics, as interpreted by the Russians in terms of the phasic theory, insisted that the individual phases of growth and development were inherited separately. On this assumption breeding should be directed to phasic analysis and combination of phasic period lengths which would fit particular environmental conditions. The criticism directed at the formal genetics maintained that the latter based its choice of gene characteristics on morphological or growth factors which were valid only in the environment of the experiment. Although Whyte (1946) expressed doubt of the validity of the rigid phasic sequences insisted upon by the Russians, he pointed out that breeding on the basis of physiological characteristics opened up new possibilities yet to be explored and tested.
According to developmental physiology, when the term adaptation to environment is used, it may involve one or more of several things such as breeding, selection, and agronomic management. The meaning of the term in this sense differs from the popular usage, and does not include the Lamarckian meaning. It is doubtful whether the individual plant can adapt itself to a change in environment. During the several phases in the growth and development of a plant, there are varying capacities to resist cold, drouth, insects, or fungus diseases. The objective would not be necessarily to breed for greater resistance to unfavorable conditions in the popular sense, because the possibilities in that direction seem to be limited. The purpose in both breeding and adaptation would be to discover and combine the growth and development factors that would afford the optimum timing to utilize favorable and avoid unfavorable conditions. This includes timing for greater resistance against insects and fungus diseases. The desired result would be achieved partly be breeding, partly be selecting individual variants (mutations), which demonstrated most effectively their ability to survive, and partly by agronomic management such as choice of planting dates. Timing of the growth and developmental phases is the point to be emphasized, not an actual change in the plant in the Lamarckian sense. This does not exclude, however, consideration of the difficult problem of hormones, and the so-called hardening process in relation to cold and drouth, which require further investigation and interpretation.
The discussion of the influence of length of day (photoperiodism) on the growth and development of plants was the contribution of Garner and Allard in 1920, and it inspired many others to study the effects of light and darkness on plants and animals. The most extensive studies of native grasses of the North American grassland in relation to drouth and light, with some reference to the temperature, are five papers by Olmstead dealing with the grama grasses (1941, 1942, 1943, 1944, 1945). The last three deal with photoperiodic responses. The third paper (1943) reported on investigation of six species of grama grass; slender, side-oats, Rothrock's, hairy, black, and blue. The experimental material for five of these came from southern Arizona, and that for the blue grama from Montana. The plants of southern origin were either short-day plants, or showed tendencies predominantly in the direction of the short-day or intermediate classes, while the blue grama from Montana seemed to be day-neutral (indeterminate), or to some extent exhibited long-day tendencies.
The fourth of the Olmstead papers (1944) reported on twelve geographic strains of side-oats grama grass. This species is found in the temperate zones of South America and as far north as 50! in North America, yet its center of distribution seems to be southwestern United States and northern Mexico. Fundamentally, therefore, it would appear to have been originally a short-day species, which had achieved a differentiation into strains with an adaptation to a wide range of light and temperature. The geographic strains used in the experiment came from different latitudes, southern Texas and Arizona to North Dakota, were morphologically and physiologically distinct and yet were members of the same species.
The fifth of the Olmstead papers (1945) reported on the photoperiodic responses to clonal divisions of three latitudinal strains of side-oats grama grass. By making clonal divisions rather than growing plants from seeds, the material of the experiment represented genetically identical populations for each of the three strains: San Antonio, Texas; El Reno, Oklahoma; and Cannonball, North Dakota. The strains from the three latitudes were adapted to the light and temperature differences of their geographical origin, exhibiting the capacity for seasons of long vegetative growth in the southern material and of short vegetative growth in the northern material, combined in each case with the capacity for development toward reproduction and maturing of seeds. The widest range of variability in photoperiodic responses was found in the strain from the middle latitude of Oklahoma. This led to the suggestion that on this basis acclimatization of this strain might be possible both north and south of the native latitude of the strain. Investigations of the blue grama grass have been made, but have not been published, and investigation of other species when undertaken should make important contributions to the understanding of the distribution of native grasses.
As applied to agricultural crops, developmental physiology has already contributed much information for the guidance of the agronomist. Although corn is a short-day plant its cultivation has been pushed northward, especially for fodder. There the day length permits vegetative growth, but the short season does not permit development for reproduction and ripening of seed. As a matter of agronomic management of domestic crops, the seed can be produced further south. Sugar beets afford a striking example of the possibility of making choices. Sugar beet seed is grown in the Pacific Northwest latitudes favorable to reproduction and the root crop is grown in southern California and Arizona. The bio-chemical composition of sugar beets may be controlled in some degree to bring the sugar content of its highest peak through limitation of irrigation water at the proper stage of growth. As a matter of practical pasture management, the Bluestem Pasture owners of Kansas recognized what plant physiologists later demonstrated as a common occurrence, that the nutritive peak of vegetative growth occurs during the transition to seed production, and diminishes rapidly as the seed forms and matures. Commercial pasture contracts run accordingly from April 1, to October 1, and out-shipments of grass-fed cattle begin in late July leaving the pastures vacant by the end of September. Similarly, bluestem grass is cut for hay in late July, prior to seed formation. The short grasses of the great plains behave differently, however, the leaves curing on the plant and retaining high nutritive value for winter pasture, a fact that was early recognized in relation to the beginnings of the livestock industry of that region.
The small grains, including wheat, are mostly long-day plants. Hard spring wheats occupy the most northerly position on the western grassland, and the hard winter wheats occupy the central area. Wheats do not find the southern grassland suitable, and there upland cotton, a short-day, warm-temperature plant, provides the staple crop. During the early years of the twentieth century, M. A. Carleton, under the auspices of the USDA imported durum (macaroni) wheats from southern Asia, recommending them especially for the southern portion of the grassland. In practice, however, the durum wheats were successful only on the northern parts of the grassland, where, among other things, they were subject to long-day conditions.