Research methods in ecology

by Frederic E. Clements

METHOD OF CONTROL CULTURES

195. Scope and procedure. Control experiments are necessarily carried on in the planthouse, since factors can be controlled in the field only with great difficulty. Their greatest value is in connection with experiments that are being carried on in the habitat, but they also constitute an invaluable means of independent research, since it is not at all difficult to approximate the conditions of a habitat, especially with reference to water-content and light. The essential feature of the method is that the less important factors are equalized as far as possible, while the direct factors, water-content and light, are under the complete control of the investigator. By the equalization of humidity and temperature is meant experimentation in which all the plants of each experiment are subjected to the same amounts of these factors. It is a matter of no importance whatever whether the humidity and temperature are constant or variable. In the case of soil, which is not a variable, it naturally happens that the plants are placed once for all in the same soil mixture. Batteries consisting of thermograph and psychrograph have been kept in the different control houses, but although used at first to give some idea of the hourly and daily fluctuations of temperature and humidity, they have slight bearing upon the evolution of new forms under control. For use in connection with supplementary experiments in adjustment and adaptation, the batteries have proved to be indispensable. Control experiments are regularly made in series which are planned with reference to as many modifications as the efficient difference of the factor and the plasticity of the species concerned permit.

196. Water-content series. An account of the experiments which have been carried on for four generations with Ranunculus sceleratus will serve to show the application of culture methods to the origin of new forms in response to varying water-content. This species was chosen because it grows readily in the planthouse, is plastic, and, since it is naturally amphibious, permits of much modification in both directions. The smallest amount of water per day under which the seedlings would grow was found to be 25 cc. This was taken as one extreme for the series, and deep water in which the plant could be submerged as the other. An arbitrary series was tentatively made as follows: 25 cc., 50 cc., 100 c., 150 cc., 200 cc., mud, shallow water, and deep water. Further study justified these divisions, since the first six gave efficient differences in water-content, and the resulting forms all showed differences of structure as well as of growth and form. Seedlings of the same age, and as nearly alike as possible, were transplanted to large pots of which there were four for each of the first six; they were placed in half-barrels for mud and floating forms, and in a barrel for submerged forms. After a few days, when they had become well established, the plants in the pots were watered in the amounts indicated, as often as was necessary to keep the most xerophytic form alive; the soil for the mud form was kept covered with a thin film of water; the leaves of the form in shallow water were kept floating on the surface, and those of the last form submerged just below the surface. The water in which the submerged form grew was aerated by means of a spigot near the bottom of the barrel. From time to time water-content determinations were made of the soil in the pots until it was definitely ascertained that the holard was practically constant. The nine new forms obtained by adaptation showed striking differences in vigor and growth, as may be seen from the figures. In all cases, these were accompanied by distinct and often striking differences in the number and position of the stomata, the amount of sponge and palisade tissues, and the development of air passages. Photographs were made of a typical plant of each form, and the different leaf structures were preserved in permanent mounts. The xerophytic and the submerged form were unable to produce flowers, and it was necessary to develop them anew in each generation. The other forms fruited abundantly, and the succeeding generations of each form were produced from plants which had grown the year before in the same conditions. In addition to the development of a series of new water-content forms, this experiment was begun in the hope of determining whether the modifications of a plastic species tend to become fixed if each new form is grown constantly under the same conditions. A period of four years is too short, however, to throw much light upon this problem.

Fig. 49. Floating form of Ranunculus sceleratus grown under control.

Helianthus annuus has been used for other series of experiments, in which alkaline salts or different soils are employed to vary the water-content. These are more complex and hence are not as satisfactory as the series described above, but they are valuable for the light they throw upon the behavior of plants in similar conditions in nature. In the case of soil, however, the adaptation may be referred to water-content alone, if thoroughly leached sands and gravels are used, so that the difference is solely one of water-retaining power.

197. Light series. Cloth tents have been found the most satisfactory means of obtaining different light intensities in the planthouse. The cloth permits the air to circulate to a considerable degree, and in consequence the equalization of humidity and temperature is much more complete than in the glass houses first employed. The cloth tents, or shade tents as they are called, are cubical, each dimension being 1 meter. The series which has been most used consists of three tents: the first is made of cheesecloth and has a light value of .1; the second is of thin muslin, and has a value of .04, while the third is made of dark cambric and the light is reduced to .01. A more desirable series is one with five tents, which have approximately the following light intensities: .1, .05, .01, .007, .003. Plants grown in shade tents should be repotted as often as they will permit in order to increase the aeration of the soil. The amount of water given them must also be decreased as the shade increases. Mesophytic species give the best results in shade tents, xerophytes thrive less well, and amphibious plants do not grow at all except in the brightest light. Excellent results have been obtained with Helianthus, Taraxacum, Gaura, and Onagra, while Ranunculus sceleratus is unable to produce flowers and seeds in a light intensity of .01.

A number of important supplementary experiments have been made in connection with light tents. These do not result in the production of new forms, but they throw much light upon it. Plants have been placed in the shade tents so that certain leaves would be in the sun and others in the shade. Young leaves have been fixed at various angles with the stem, and they have been revolved 90° or 180° in order to change the relation of their surfaces. Soils of different colors, e. g., loam and sand, have been used to determine the effect of light reflected from their surfaces. Shade tents make it possible to illuminate plants from the top, bottom, or side, and to carry on a large number of fundamental experiments in adjustment and adaptation.

CHAPTER IV. THE PLANT FORMATION
Methods of Investigation and Record

198. The need of exact methods. The use of instruments in the study of the habitat has made it evident that the loose methods of descriptive ecology were altogether inadequate to the accurate investigation of the formation. This feeling has been heightened by the recognition of the fact that vegetation exhibits both development and structure, and is, in consequence, open to exact methods of inquiry. In the search for feasible methods, it was quickly seen that the quadrat, first[^[20]] used for determining the abundance of species, furnished the key to the problem. Accordingly, the principle underlying it, viz., that of intimate detailed study and record, was developed and extended in such a way as to give rise to a number of methods of precision. These have been applied in the field for several years with signal success, and they are here described in the conviction that they constitute a satisfactory system, if not, indeed, the only one for the exact study of formations.

There has been a growing appreciation of the fact that the superficial methods of descriptive ecology made it impossible to build upon such a foundation, and they, indeed, were making actual progress in the field of ecology more and more difficult. Ecologists have now begun to see clearly that precise methods are as indispensable in the habitat as they are to the study of the structure and modification of the plant. For some reason, however, they have been slow to perceive that accuracy in the investigation of the cause, the habitat, is a fruitless task unless it be followed by corresponding exactness in the study of the effect, the formation. After having urged the fundamental necessity of instrumental methods, for six or seven years, both in season and out of season, the writer does not feel called upon to further plead the cause of the quadrat. The final acceptance of the instrument was inevitable if progress were to be made in the habitat, and it is just as obvious that the quadrat must be accepted if the study of the habitat is to bear fruit in the interpretation of the formation. The use of the quadrat does not mean that the general methods of descriptive ecology are all to be discarded, whether they have value or not. The statement that quadrat methods are indispensable signifies merely that they must be used for research work in the development and structure of vegetation. They are not necessary in reconnaissance, nor do they displace general methods of real value. The use of the latter in even a supplementary way will gradually be discontinued, however, as fields become smaller by reason of increase in the number of workers, and as the need for precise methods becomes more universally felt.

The quadrat constitutes the initial concept from which all the methods have grown. In itself, it has given rise to a variety of quadrats applicable to the most fundamental problems of vegetation. From it have come, on the one hand, the migration circle, and on the other, the transect. The latter in turn has yielded the ecotone chart, and the layer chart. All of these are based upon direct and detailed contact with vegetation itself, and permit accurate recording of all the results obtained.