141. Combinations of curves are invaluable for bringing similar curves together, and permitting ready comparison of them. For this, and also because they save space, they are regularly employed to the almost complete exclusion of single curves. Combinations are made simply by tracing the curves to be compared upon the same sheet, it being understood that dissimilar curves, e. g., level and station, can not be combined. Colored inks are an absolute necessity in combining; the primary principle underlying their use is that curves that approach closely or cross each other must be traced in inks that contrast sharply. As elsewhere stated, it has been made the invariable rule to use the same color for the same level or point. This applies especially to temperature, but holds also for humidity, light, wind, and water-content, so that the color always indicates the level. For the same reason, it is applied to a combination of point curves for one station, though it is inapplicable to a series of point curves when these lie in the same level. Light readings above 6 feet and water-content readings below 15 inches necessitate the use of additional colors.
Combinations may be made of the curves of a single factor for purposes of comparison, or they may consist of curves of different factors in order to aid in interpreting or indicating their relation to each other. Curves of the same factor may be combined to form various series. The level series consists of all the level curves for the stations under observation, e. g., the six levels for temperature, three levels for wind, etc. Similarly, the station series is a combination of all the station curves, and a corresponding arrangement may be made for point curves with reference either to station or to level. An extremely valuable combination of curves is that of the holard and chresard for a series of stations. The most important combinations of the curves of different factors are naturally those based upon factors intimately related to each other or to the plant. The grouping of water-content and humidity curves is of great value, especially when the transpiration curve is added. Light and temperature curves make an interesting combination, while a humidity, temperature, and wind series is of much aid in tracing the connection between these factors. Finally, it is altogether feasible to arrange the curves of water-content, humidity, light, temperature, and wind upon the same sheet in such fashion as to give a graphic representation of the whole physical nature of a single habitat or a series. In all combinations of curves representing different factors, it must be borne in mind that the position of a curve does not represent a definite value with reference to the others, since some are based upon per cents, others upon degrees, etc. The comparison must be based upon the character of the curves, but even then it is an important aid. An instructive grouping has been employed where series of readings on the same day, or on two successive days in forest and in prairie have yielded the usual level series of curves. The series for the two habitats are arranged on the same page, one at the right and the other at the left, and permit direct comparison of corresponding level or factor curves, both with respect to position and character.
142. The amplitude of all the curves described above is determined by the unit values of the factors concerned, while the length is dependent upon the number of stations, points, or times. The value assigned the latter upon the plotting paper is purely arbitrary, but it is most convenient to fix this at the centimeter square. The unit value for temperature is 1° Centigrade per square, each subdivision of the latter representing 0.2, and the range being 22 degrees. For water-content curves, each square represents a value of 2 per cent, the smaller square being 0.4 per cent, and the range 2–48 per cent. The unit value for humidity is taken as 5 per cent, making each small square 1 per cent, and giving room on the sheet for the entire range from 1–100 per cent. Owing to the anemometer used, curves of wind velocity have been based upon the number of feet per minute. One hundred feet is taken as the unit value, and the range is from 0–2200 feet. The unit value for the curve of light intensity is .005. Each small square is .001, which permits a range from .001 to .01 on one sheet. Consequently, when it is desired to plot the curve of a series of habitats with a range in intensity greater than this it is necessary to use a double sheet. This is the usual device when the range of curves is too great, except where the excess is slight. In this case the curve is left open at the top, and the value which the crest attains is indicated. All curves in combination are labeled at the beginning or left to indicate the level, station, or point, and at the end or right to show the time, or day, if this is not the basis of the curve or series.
The discussion that precedes deals exclusively with curves representing factors determined in the field. It applies with equal force to results obtained by instruments in control houses. In these, however, all factors except those directly experimented with, usually water-content and light, are practically equalized, and the curves based upon them are used chiefly to show how nearly equal they have become. The important curves are those of the water-content series, both holard and chresard, and of the shade tents. Where several houses are differentiated with respect to temperature or humidity, curve series of both these factors are necessary.
Factor Means and Sums
143. It has been shown elsewhere that the daily mean of temperature can be closely approximated from the maximum and minimum of both day and night. Maximum-minimum instruments for the other factors are lacking, however, and for light, humidity, and wind these values can only be obtained from the ecograph which makes it possible to get the exact mean from the sum of all the hour readings. When it comes to the seasonal mean, the ecograph is even more necessary, exception being made for water-content, in which case a number of readings on various days through the season will suffice. The value of factor means for diagnosis and for curves has already been sufficiently commented upon, and the feasibility of factor sums already indicated.
CHAPTER III. THE PLANT
Stimulus and Response
GENERAL RELATIONS
144. The nature of stimuli. Whatever produces a change in the functions of a plant is a stimulus. The latter may be a force or a material; it may be imponderable or ponderable; effect, not character, determines a stimulus. Consequently, reaction or response decides what constitutes a stimulus. The presence of the latter can be recognized only through an appreciable or visible response, since it is impossible to discriminate between an impact which produces no reaction and one which produces a merely latent one. From this it is evident that quantity is decisive in determining whether the impact becomes a stimulus. Plants grow constantly under the influence of many stimuli, all varying from time to time in amount. Small changes in these are so frequent that, in many cases at least, the plant no longer appreciably reacts to them. Such changes, though usually measurable, are not stimuli. Furthermore, it must be clearly recognized that plants which are in constant response to stimuli are stimulated anew by an efficient increase or decrease in the amount of any one of these. As is well known, however, such increase or decrease is a stimulus only within certain limits, and the degree of change necessary to produce a response depends upon the amount of the factor normally present. The entire absence of a force usually present, moreover, often constitutes a stimulus, as is evident in the case of light. The nature of the plant itself has a profound bearing upon the factors that act as stimuli. Many species are extremely labile, and react strongly to relatively slight stimuli; others are correspondingly stable, and respond only to stimuli of much greater force. Some light is thrown upon the nature of this difference by the behavior of ecads. A form which has grown under comparatively uniform conditions for a long time seems to respond less readily, and is therefore less labile than one which is subject to constant fluctuation. In many cases this is not true, however, and the degree of stability, i. e., of response, can only be connected in a general way with taxonomic position.
145. The kinds of stimuli. The factors of a habitat are external to the plant, and consequently are termed external stimuli. Properly speaking, all stimuli are external, but since the response is often delayed or can not be clearly traced, it may be permissible to speak of internal stimuli, i. e., those which appear to originate within the plant. These, however, are extremely obscure, and it is hardly possible to deal with them until much more is known of the action of external stimuli. Of the latter, certain forces, gravity and polarity, act in a way not at all understood, and as they are essentially alike for all plants and all habitats, they can here be ignored. Stimuli are imponderable when, like light and heat, they are measured with reference to intensity, and ponderable, when, as in the case of water-content, humidity, and salt-content, they can be expressed in mass or weight. It is undesirable to insist upon this distinction, however, since the real character of a stimulus is determined by its effect, and the latter is not necessarily dependent upon whether the stimulus is one of force or one of material. There is, however, a fundamental difference between factors with respect to their relation to the plant. Direct factors alone are stimuli, since indirect factors must always act through them. For example, the wind, its mechanical influence excepted, can affect the plant only in so far as it is converted into the stimulus of increased or decreased humidity. Consequently, the normal stimuli of the plants of a formation are: (1) water-content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. Soil, pressure, physiography, and biotic factors influence plants only through these, and are not stimuli, though exceptions must be made of biotic factors in the case of sensitive, insectivorous, and gall-producing plants.