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PRESENT PROBLEMS OF PHYSIOLOGICAL

PLANT ECOLOGY

DR. BURTON E. LIVINGSTON

DESERT BOTANICAL LABORATORY

By physiological ecology is here meant merely the study of the factors which determine the occurrence and behavior of plants growing under uncontrolled conditions. Field physiology would be almost a synonym for the term here used. Pure physiology proceeds, as far as is possible, by controlling conditions. By varying known conditions and measuring the plant responses definite relations are established between stimulus and response. But ecology must perforce proceed without the known conditions, without the synthetically built-up environment of pure physiological research. Here it is necessary to measure and analyze natural conditions and to relate these to the behavior of the plants. The problem of measuring the plant phenomena, the determination of the number, size, form, structure, etc., of the plants involved, is essentially the same for both lines of study, but that of measuring environmental conditions is, of course, much more difficult where the latter are uncontrolled.

There is a principle of scientific research, that in an investigation which involves the measurement of a number of causal factors and the relation of these factors to resulting conditions, the various measurements should be of as nearly equal accuracy as possible. Where a number of complex factors are to be dealt with, as in ecology, progress must come, on the one hand, from a refinement of methods of measurement and, on the other, from better interpretations of the data resulting from these measurements. According to the principle just stated, we should ever seek to improve those of our methods which are the

least accurate, not those which are the most easily susceptible of improvement. It is, indeed, often a waste of energy to seek the highest possible accuracy in all of a series of measurements where one or more are at best of low accuracy. The accuracy of the resulting summation must be subject roughly to the error of the least accurate of the members.

It is not my purpose to submit any recommendations. as to improvement in the general philosophy of ecology, although we must all realize that one of our greatest hindrances at present lies in the careless thinking which fills our literature with wrong or at least misleading imaginings, such as are suggested by the Jonah-like words, adaptation, use, purpose, etc. It is to certain lines along which improved methods of measurement seem desirable that I wish to call your attention at the present time. Our methods for dealing with the plant responses, with the effects of environment, already possess a general accuracy far surpassing that exhibited by the methods employed in measuring the environmental conditions which act as causes. I am unable to avoid mentioning, however, one phase of plant measurement which has so far received an almost insignificant minimum of attention, both from physiologists and ecologists. I refer to the subterranean portions of the plant. The relations existing between the environmental factors and the development and behavior of underground stems have been studied for some forms by Vöchting, Klebs and others. Roots have usually been studied in water or in air and only in the last few years has their behavior in the natural medium, the soil, been seriously taken up. A number of agriculturists have attacked the problem here suggested, but of course with reference to cultivated plants. Freidenfeld has made, as far as I am aware, the first attempt at a really broad study of the ecology of roots. The work of Dr. Cannon on the root systems of desert plants shows how extremely important subterranean competition may sometimes be. It would seem

that the underground portions of ordinary plants are well worthy of more attention than has heretofore been accorded them.

We turn now to the environmental factors. When it is sought to determine the causes influencing the behavior of plants growing under natural conditions, two very different methods may be resorted to, the observational and the experimental. By the experimental method we try to determine, on the one hand, the kind and amount of vegetation and, on the other, the magnitude of the various physical conditions which make up the environment. From these two series of observations are selected paired concomitant factors or groups of factors, and when the same concomitants appear in a number of such pairs the conclusion is drawn that the relation between the paired factors is a causal one. By the experimental method we seek to control the conditions to a greater or less degree, either synthetizing an artificial environment (experimental physiology), or growing the same plant forms under various natural environmental complexes. This sort of work may be termed experimental ecology. In so far as the environment is artificially synthetized, it is comparatively easy of measurement, but where natural conditions are allowed to obtain, the experimental and the observational methods both require the measurement of uncontrolled factors, and thus present great difficulties.

The complexity of natural conditions makes it necessary often to break them up into component parts and to measure the parts separately. For convenience in handling, these factors have been classified into climatic and edaphic, but I fail to see that such a classification has any relation to the activities of the plant. I shall, therefore, speak here merely of environmental factors, classifying them, for ease in discussion, into those which are active above the soil surface and those which are active below it. Each group must, of course, be further analyzed according to the purposes of the investigation. But it must be remembered that the data from separate com

ponent factors usually need to be again summed in order to express the environment as a whole. For the great general problems of plant geography it seems inadvisable to attempt too extended an analysis, rather is it better to seek methods of measurement which will furnish integrated evaluations of groups of environmental conditions. With our present lack of knowledge, the pressing of the analysis too far often results in such a complex of data that an interpretation is impossible. A fairly satisfactory integration of the main air factors seems to be furnished by the atmometer; as to the soil factors, we have made hardly a beginning in this direction. In the following paragraphs I shall first consider the measurement of the air factors, denoting by this term all factors which are active above the soil surface, whether or not the air is actually involved.

Atmospheric Factors.-Atmospheric pressure can be easily measured by means of the barometer, curves are automatically constructed by the barograph. Considering the perfection of this instrument, it is rather unfortunate for ecology that the plant is so little influenced by the natural variations in atmospheric pressure.

Temperature is very important in plant activities, and we have practically perfect instruments for its measurement and for the construction of its curve. Unfortunately we have as yet no well-tested method by which temperature can be interpreted in regard to its effect. A beginning which promises much has been made by Dr. MacDougal with his integration of the thermograph record and Professor Lloyd has told me of a new method devised by him for interpreting maxima and minima. Here lies one of the best fields for the scientific ecologist with a mathematical turn of mind.

Wind velocity can be measured and recorded by means of the ordinary forms of anemometer, but the instruments are not well suited to field work, largely on account of their expense. Perhaps improvement may be forthcoming along this line.

The conditions of humidity, which appear to be so important to plant life, can best be measured directly by means of the dew-point apparatus, but the instrument is not as satisfactory in the operation as could be desired. The whirled psychrometer and the wet and dry bulb thermometer are more easily operated and are quite satisfactory as regards results, especially where the humidities dealt with are not too low. Attention may be called here, however, to the inadequacy of the wet and dry bulb thermometer without a strong current of air. The current should be so strong that any increase in its velocity would produce no further lowering of the temperature of the wet bulb. The hair hygrometer is unreliable unless often standardized by some other instrument. Especially is this so in regions where the humidity is usually low or where its fluctuations are very great. Much improvement is surely possible in connection with this factor.

The evaporating power of the air, an integration of the effects of temperature, wind velocity, relative humidity, and, to some extent, of light intensity, is at least not as difficult of measurement as formerly. The porous cup atmometer can be made to give data for a curve as well as a final integration for a long period. It seems that we may expect much from this or some similar instrument.

Precipitation data are easy to obtain with amply sufficient accuracy, but the factors of superficial and subterranean run-off as well as that of evaporation from the soil surface (all of which are almost hopelessly difficult to measure) make these data very hard to interpret, except for particular localities. Their final interpretation will probably go hand in hand with that of evaporation and soil moisture.

For the measurement of light intensity-a factor which has been shown conclusively to be of prime importance in many ecological problems-we have at present no reliable and practical instrument. The delicate bolometer

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