TEMPERATURE

Fig. 13. Chart for the determination of the sun’s altitude, and the corresponding light intensity.

The actual differences in the light intensity from hour to hour and day to day, which are caused by variations in the sun’s altitude, are not as great as might be expected. For example, the maximum intensity at Lincoln, June 22, is .98; the minimum meridional intensity December 22 is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00A.M.and 6:00P.M.approximately); between 8:00A.M.and 4:00P.M.the range in intensity is from .90 to .98 merely. On December 22, the greatest intensity is .52, the least .20 (the latter at 8:00A.M.and 4:00P.M.approximately). If the growing season be taken as beginning with the 1st of March and closing the 1st of October, the greatest variation in light intensity at Lincoln within a period of 10 hours with the meridian at its center (cloudy days excepted) is from .33 to .98. In a period of 8 hours, the extremes are .65 to .98, i. e., the greatest variation, .3, is far within the efficient difference, which has been put at .9. For the growing period, then, readings made between 8:00A.M.and 4:00P.M.on normal sunshiny days may be compared directly, without taking into account the compensation for the sun’s altitude. Until the efficient difference has been determined for a large number of species, however, it seems wise to err on the safe side and to compensate for great differences in time of day or year. In all doubtful cases, the intensity obtained by the astronomicalmethod should also be checked by photometric readings. A slight error probably enters in, due to reflection from the surface of the paper, and to temperature, but this is negligible.

86.Table for determining apparent noon

87. Place.The effect of latitude upon the sun’s altitude, and the consequent light intensity have been discussed in the pages which precede. Latitude has also a profound influence upon the duration of daylight, but the importance of the latter apart from intensity is not altogether clear. The variation of intensity due to altitude has been greatly overestimated; it is practically certain, for example, that the dwarf habit of alpine plants is not to be ascribed to intense illumination, since the latter increases but slightly with the altitude. It has been demonstrated astronomically that about 20 per cent of a vertical ray of sunlight is absorbed by the atmosphere by the time it reaches sea level. At the summit of Pike’s Peak, which is 14,000 feet (4,267 meters) high, the barometric pressure is 17 inches, and the absorption is approximately 11 per cent. In other words, the light at sea level is 80 per cent of that which enters the earth’s atmosphere; on the summit of Pike’s Peak it is 89 per cent. As the effect of the sun’s altitude is the same in both places, the table of curves on page57will apply to both. Taking into account the difference in absorption, the maximum intensity at sea level and at 14,000 feet on the fortieth parallel is .98 and 1.09 respectively. The minimum intensities between 8:00A.M.and 4:00P.M.of the growing period are .64 and .71 respectively. The correctness of these figures has been demonstrated by photometer readings, which have given almost exactly the same results. Such slight variations are quite insufficient to produce an appreciable adjustment, particularly in structure. They are far within the efficient difference, and Reinke[5]has found, moreover, that photosynthetic activity inElodeais not increased beyond the normal in sunlight sixty times concentrated. In consequence, it is entirely unnecessary to take account of different altitudes in obtaining light values.

The slope of a habitat exerts a considerable effect upon the intensity of the incident light. If the angle between the slope and the sun’s ray be 90°, a square meter of surface will receive the maximum intensity, 1. At an angle of 10°, the same area receives but .17 of the light. This relation between angle and intensity is shown in the table which follows. The influence of the light, however, is felt by the leaf, not by the slope. Since there is no connection between the position of the leaf and the slope of the habitat, the latter may be ignored. In consequence, it is unnecessary to make allowances for the direction of a slope, viz., whether north, east, south, or west, in so far as light values are concerned. The angle which a leaf makes with its stem determines the angle of incidence, and hence the amount of light received by the leaf surface. This is relatively unimportant for two reasons. This angle changes hourly and daily with the altitude of thesun, and the intensity constantly swings from one extreme to the other. Moreover, the extremes 1.00 and 0.17, even if constant, are hardly sufficient to produce a measurable result. When the angle of the leaf approaches 90°, there is the well-known differentiation of leaf surfaces and of chlorenchym, but this has no relation to the angle of incidence.

In the sunlight, it makes no difference at what height a light reading is taken. In forest and thicket as well as in some herbaceous formations, the intensity of the light, if there is any difference, is greatest just beneath the foliage of the facies. In forests especially, the light is increasingly diffuse toward the ground, particularly where layers intervene. In woodland formations, moreover, the exact spot in which a reading is made must be carefully chosen, unless the foliage is so dense that the shade is uniform. A very satisfactory plan is to take readings in two or more spots where the shade appears to be typical, and to make a check reading in a “sunfleck,” a spot where sunlight shows through. In forests and thickets, the sunflecks are fleeting, and the light value is practically that of the shade. In passing into open woodland and thicket, the sunflecks increase in size and permanence, until finally they exceed the shade areas in amount and become typical of the formation.

88. The fate of incident light.The light present in a habitat and incident upon a leaf is not all available for photosynthesis. Part is reflected or screened out by the epidermis, and a certain amount passes through the chlorenchym, except in very thick leaves. The light absorbed is by far the greatest in the majority of species. Many plants with dense coatings of hairs reflect or withhold more light than they absorb, and the amount of light reflected by a thick cuticule is likewise great. As light is imponderable, the actual amount absorbed or reflected by the leaf can not be determined. It is possible, however, to express this in terms of the total amountreceived, by means of readings with solio paper, and the knowledge thus obtained is of great importance in interpreting the modifications of certain types of leaves. For example, a leaf with a densely hairy epidermis may receive light of the full intensity, 1; the amount reflected or screened out by the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the majority of cases, however, the absorbed light is considerably more than the amount reflected or transmitted.

Fig. 14. Leaf print: exposed 10 m., 11A.M.August 20. The leaves are from sun and shade forms ofBursa bursa-pastoris,Rosa sayii,Thalictrum sparsiflorum, andMachaeranthera aspera. In each the shade leaf prints more deeply.

89. Methods of determination.If results are to be of value, reflected and transmitted light must be determined in the habitat of the plant simultaneously with the total light which a leaf receives. An approximation of the light reflected from a leaf surface is secured by placing the photometer so that the light reflected is thrown upon the solio strip. A much more satisfactory method, however, is to determine it in connection with the amount of light transmitted through the epidermis. This is done by stripping a piece of epidermis from the upper surface of the leaf and placing it over the slit in the photometer for an exposure. An exposure in the full light of the habitat is made simultaneously with another photometer, or immediately afterward upon the same strip. When the epidermis is not too dense, both exposures are permitted to reach the same tint, and the relation between them is precisely that of their lengths of exposure. Ordinarily the two exposures are made absolutely simultaneous by placing the epidermis over half of the opening, leaving the other half to record the full light value, and the results, orepidermis prints, are referred to a multiplestandard. The difference between the two values thus obtained represents the amount of reflected light together with that screened by the epidermis. The amount of light transmitted through the leaf may be measured in the same way by using the leaf itself in place of the epidermis alone. The time of exposure is necessarily long, however, and it has been found practicable to obtain leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time that the epidermis print is made. In a few species both the upper and lower epidermis can be removed and the amount of light absorbed determined directly by exposing the strip covered with the chlorenchym. Generally, however, this must be computed by subtracting the sum of the per cents of reflected and transmitted light from 100 per cent, which represents the total light.

Fig. 15. Leaf print: exposure as before. Sun and shade leaves ofAchillea lanulosa,Capnoides aureum,Antennaria umbrinella,Galium boreale, andPotentilla propinqua.

90. Leaf and epidermis prints.In diphotic leaves the screening effect of the lower epidermis may be ignored. Isophotic sun leaves, i. e., those nearly upright in position or found above light-colored, reflecting soils, are usually strongly illuminated on both sides, and the absorbed light can be obtained only by measuring the screening effect of both epiderms. Shade leaves and submerged leaves often contain chloroplasts in the epidermis, and the above method can not be applied to them. In fact, in habitats where the light is quite diffuse, practically all incident light is absorbed. The rare exceptions are those shade leaves with a distinct bloom. In addition to their use in obtaining the amount of light absorbed, both leaf and epidermis prints are extremely interesting for the direct comparison of light relations in the leaves of species belonging to different habitats. The relative screening value of the upper and lower epidermis, or of the corresponding epiderms of two ecads or two species, is readily ascertained by exposing the two side by side in sunshine, over the slit in the photometer. For leaf prints fresh leaves are desirable, though nearly the same results can be obtained fromleaves dried under pressure. The leaves are grouped as desired on the glass of a printing frame, and covered with a sheet of solio. They are then exposed to full sunlight, preferably at meridian, and the prints evaluated by means of the multiple standard. This method is especially useful in the comparison of ecads of one species. These differences due to transmitted light are very graphic, and can easily be preserved by “toning” the print in the usual way.

91. Light records.The actual photographic records obtained by photometer and selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning” modifies the color of the exposure materially, and changes its intensity so that it can not be compared with readings not fixed. It would involve a great deal of inconvenience to make all comparisons by means of toned strips and standard, even if it were not for the fact that it is practically impossible to obtain exactly the same shade in lots toned at different times. The field record, if carefully and neatly made, may well take the place of a permanent one. The form is the following:

DayHourFormationStationAltitudeExposureGroupHeightNo.Length of exposureStandardLight valueBase valueReflected lightTransm’d lightAbsorbed light14/9/0412:00M.SpruceMilky Way2600 m.N.E. 20°Opulaster1 foot2:10160 s.3 s.019„12:05P.M.SpruceMoss Glen2500 m.LevelStreptopus„2:12240 s.3 s..012„12:15P.M.Brook b’nkGrotto2500 m.E. 3°FilixSurface2:13360 s.3 s..008

92. Light sums, means, and curves.Owing to the fact that the selagraph has not yet been used in the field, no endeavor has been made to determine the light value for every hour of the day in different habitats. Consequently there has been no attempt to compute light sums and means. Photometer readings have sufficed to interpret the effect of light in the structure of the formation, and of the individual, but they have not been sufficiently frequent for use in ascertaining sums and means. The latter are much less valuable than the extremes, especially when the relative duration of these is indicated. Means, however, are readily obtained from the continuous records. Light sums are probably impracticable, as the factor is not one that can be expressed in absolute terms. The various kinds and combinations of light curves are essentially the same as for humidity. The level curve through a series of habitats is the most illuminating, but the day curve of hour variations is of considerable value. The curve ofdaily duration, based upon full sunlight, is also of especial importance for plants, and stations which receive both sun and shade during the day.

93.In consequence of its indirect action, temperature does not have a striking effect upon the form and structure of the plant, as is the case with water and light. Notwithstanding, it is a factor of fundamental importance. This is especially evident in the character and distribution of vegetation. It is also seen in the germination and growth of plants, in the length of season, and in the important influence of temperature upon humidity, and hence upon water-content. Because of its intimate relation with the comfort of mankind, the determination of temperature values has received more attention than that of any other factor, and excellent simple and recording instruments are numerous. For plants, it is also necessary to employ instruments for measuring soil temperatures. The latter unquestionably have much less meaning for the plant than the temperatures of the air, but they have a direct influence upon the imbibition of water, and upon germination.

94. Air thermometers.The accurate measurement of temperature requires standard thermometers. Reasonably accurate instruments may be standardized by determining their error, but they are extremely unsatisfactory in practice, since they result in a serious waste of time. Accurate thermometers which read to the degree are entirely serviceable as a rule, but instruments which read to a fraction of a degree are often very much to be desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are delicate and require careful handling, but even in class work this has proved to be an advantage rather than otherwise. In making readings of air temperatures with such thermometers, constant precautions must be taken to expose the bulb directly to the wind and to keep it away from the hand and person.

95. Soil thermometers.The thermometer described above has been used extensively for soil temperatures. The determination of the latter is conveniently combined with the taking of soil samples, by using the hole for a temperature reading. When carefully covered, these holes can be used from day to day throughout the season without appreciable error, even ingravel soils. Repeated tests of this have been made by simultaneous readings in permanent and newly made holes, and the results have always been the same. It has even been found that the error is usually less than 1 degree when the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in the fact that the thermometer must be raised to make the reading. With a little practice, however, the top of the column of mercury may be raised to the surface and read before the change of temperature can react upon it. This is especially important in very moist or wet soils where the bulb becomes coated with a film of moisture. This evaporates when the bulb is brought into the air, and after a moment or two the mercury slowly falls.

Fig. 16. Soil thermometer

Regular soil thermometers are indispensable when readings are desired at depths greater than 12–18 inches. They possess several disadvantages which restrict their use almost wholly to permanent stations. It is scarcely possible to carry them on field trips, and the time required to place them in the soil renders them practically useless for single readings. Moreover, the instruments are expensive, ranging in price from $7 for the two-foot thermometer, to $19 for the eight-foot one. When it is recognized that deep-seated temperatures are extremely constant and that the slight fluctuations affect, as a rule, only the relatively stable shrubs and trees, it is evident that such temperatures are of restricted importance. Still, in any habitat, they must be ascertained before they can well be ignored, though it is unwise to spend much time and energy in their determination. Soil thermometers of the form illustrated may be obtained from H. J. Green, Brooklyn.

96. Maximum-minimum thermometers.These are used for determining the range of temperature within a given period, usually a day. Since they are much cheaper than thermographs, they can replace these in part, although they merely indicate the maximum and minimum temperatures for the day, and do not register the time when each occurs. The maximum is a mercurial thermometer with a constriction in the tube just above the bulb; this allows the mercury to pass out as it expands, but prevents it from running back, thus registering the maximum temperature. The minimum thermometer contains alcohol. The column carries a tiny dumbbell-shaped marker which moves down with it, but will not rise asthe liquid expands. This is due to the fact that the fluid expands too slowly to carry the marker upward, while the surface tension causes it to be drawn downward as the fluid contracts. The minimum temperature is indicated by the upper end of the marker. In setting up the thermometers, they are attached by special thumbscrews to a support which holds them in an oblique position. The minimum is placed in a special holder above the maximum which rests on a pin that is used also for screwing the pivot-screw into position. The support is screwed tightly to the cross-piece of a post, or in forest formations it is fastened directly to a board nailed upon a tree trunk. A shelter has not been used in ecological work, although it is the rule in meteorological observations. The minimum thermometer is set for registering by raising the free end, so that the marker runs to the end of the column. The mercury of the maximum is driven back into the bulb by whirling it rapidly on the pivot-screw after the pin has been taken out. This must be done with care in order that the bulb may not be broken. As soon as the instrument comes to rest, it is raised and the pin replaced, great care being taken to lift it no higher than is necessary. When the night maximum is sought, the thermometer should be whirled several times in order to drive the column sufficiently low. Usually, in such cases, a record is made of this point to make sure that the maximum read is the actual one. If the pivot-screw is kept well oiled, less force will be required to drive the mercury back. In practice, the thermometers have been observed at 6:00A.M.and 6:00P.M.each day, thus permitting the reading of the maximum-minimum for both day and night. Pairs of maximum-minimum thermometers are to be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, Baltimore, Maryland, at a cost of $8.25.

Fig. 17. Maximum-minimum thermometer.

Fig. 18. Terrestrial radiation thermometer.

Fig. 19. Draper thermograph.

97. Radiation thermometers.These are used to determine the radiation in the air, and from the soil, i. e., for solar and terrestrial radiation. The latter alone has been employed in the study of habitats, chiefly for the purpose of ascertaining the difference in the cooling of different soils at night. The terrestrial radiation thermometer is merely a special form of minimum thermometer, so arranged in a support that the bulb can be placed directly above the soil or plant to be studied. It is otherwise operated exactly like the minimum thermometer, and the reading gives the minimum temperature which the air above the plant or soil reaches,notthe amount of radiation. As a consequence, these instruments are valuable only where read in connection with a pair of maximum-minimum thermometers in the air, or when read in a series of instruments placed above different soils or plants.

98. Thermographs.Two types of standard instruments are in general use for obtaining continuous records of air temperatures. These are the Draper thermograph, made by the Draper Manufacturing Company, 152 Front St., New York city ($25 and $30), and the Richard thermograph sold by Julien P. Friez, Baltimore ($50). After careful trial had demonstrated that they were equally accurate, the matter of cost was considered decisive, and the Draper thermograph has been used exclusively in the writer’s own work. This instrument closely resembles the psychrograph manufactured by the same company. It is made in two sizes, of which the larger one is the more satisfactory on account of the greater distance between the lines of the recording disk. The thermometric part consists of two bimetallic strips, the contraction and expansion of whichare communicated to a hand carrying a pen. The latter traces a line on the record sheet which is attached to a metal disk made to revolve by an eight-day clock. In practice the thermograph is set up in the shelter which contains the psychrograph, and in exactly the same manner. The clock is wound, the record put in place, and the pen inked in the same way also. The proper position of the pen is determined by making a careful thermometer reading under the shelter, and then regulating the pen hand by means of the screws at the base of it. A similar test reading is also made each week, when the clock is rewound. A record sheet may be left in position for three weeks, the pen being filled each week with a different ink. The fixed order of using the inks is red, blue, and green as already indicated.

Fig. 20. Shelter for thermograph.

Owing to the fact that they are practically stationary, soil thermographs are of slight value, except at base stations. Here, the facts that they are expensive, that the soil temperatures are of relatively little importance, and that they can be determined as easily, or nearly so, by simple thermometers, make the use of such instruments altogether unnecessary, if not, indeed, undesirable. In a perfectly equipped research station, they undoubtedly have their use, but at ordinary stations, and in the case of private investigators, their value is in no wise commensurate with their cost.

Readings

99. Time.The hourly and daily fluctuations of the temperature of the air render frequent readings desirable. It is this variation, indeed, which makes single readings, or even series of them, inconclusive, and renders the use of a recording instrument almost imperative in the base station at least. Undoubtedly, a set of simultaneous readings at different heights in one station, or at the same height in different stations, especially if made at the maximum, have much value for comparison, but their full significance is seen only when they are referred to a continuous base record. Such series, moreover, furnish good results for purposes of instruction. In research work, however, it has been found imperative to have thermographs in habitats of widely different character. With these as bases, it is possible to eke them out with considerable satisfaction by means of maximum-minimum thermometers in less different habitats, or in different parts of the same habitat. Naturally these are less satisfactory, and are used only when expense sets a limit to the number of thermographs. In a careful analysis of a single habitat, more can be gained by one base thermograph supplemented by three pairs of maximum-minimum thermometers in dissimilar areas of the habitat than by two thermographs, and the cost is the same.

Fig. 21. Richard thermograph.

100. Place and height.For general air temperatures, thermograph and thermometer readings are made at a height of 3 feet (1 meter). Soil temperatures are regularly taken at the surface and at a depth of 1 foot. When a complete series of simultaneous readings is made in one station, the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 10, and 15 inches in the soil. When sun and shade occur side by side in the same formation, as is true of many thickets and forests, surface readings are regularly made in both. Similarly, valuable results are obtained by making simultaneous readings on the bare soil, on dead cover, and upon a leaf, while the influence of cover is readily ascertained by readings upon it and beneath it. A full series of station readings made at the same time upon north, east, south, and west slopes is of great importance in studying the effects of exposure.

101. Temperature records.Neither field nor permanent form is required for thermographic records, other than the record sheet itself, which contains all the necessary information in a fairly convenient form. Although the temperature of a particular hour and day can not be read at a mere glance, it can be obtained so easily that it is a waste of time to make a tabular copy of each record sheet. For thermometer readings, either single or in series, the following form is used:

DayHourFormationStationAltitudeExposureCommunityPOSITION OF READINGThermographSkyWind3 feetSurf.12 in.17/8/046:30A.M.SpruceJack Brook2550 m.N.E. 5°Mertensiare9°9°9.8°10°Clear0„„Half gravelHiawatha2550 m.N.E. 7°Asterare11.2°11.2°14.8°10°Clear0„6:30P.M.SpruceJack Brook2550 m.N.E. 5°Mertensiare11.4°11.4°9.8°11°Cloudy0„„Half gravelHiawatha2550 m.N.E. 7°Asterare12°13.8°16.4°11°Cloudy0

102. Temperature sums and means.The amount of heat, i. e., the number of calories received within a given time by a definite area of plant surface, can be determined by means of a calorimeter. From this the temperature sum of a particular period may be obtained by simple addition. In the present condition of our knowledge, it is impossible to establish any exact connection between such results and the functional or growth effect that can be traced directly to heat. As a consequence, temperature sums do not at present contribute anything of value to an understanding of the relation between cause and effect. The mean daily temperature is readilyobtained by averaging twenty-four hour-temperatures recorded by the thermograph. The method employed by Meyen[6], of deriving the mean directly from the maximum and minimum for the day, is not accurate; from a large number of computations, the error is always more than two degrees. On the other hand, the mean obtained by averaging the maximum and minimum for the day and night has been found to deviate less than 1 degree from the mean proper. This fact greatly increases the value of maximum-minimum instruments if they are read daily at 6:00A.M.and 6:00P.M.

103. Temperature curves.The kinds and combinations of temperature curves are almost without number. The simple curves of most interest are those for a series of stations or habitats, based upon the level of three feet, or the surface, or the daily mean. The curves for each station representing the different heights and depths and the season curve of the daily means for a habitat are also of much importance. One of the most illuminating combinations is that which groups together the various level curves for a series of habitats. Other valuable combinations are obtained by grouping the curves of daily means of different habitats for the season, or the various station curves.

104. Plant temperatures.The direct effects of temperature as seen in nutrition and growth can be ascertained only by determining the temperature of plant tissues. The temperatures of the air and of the soil surface have an important effect upon humidity, and water-content, and through them upon the plant, but heat can influence assimilation, for example, only in so far as it is absorbed by the assimilating tissue. The temperatures of the leaf, as the most active nutritive organ of the plant, are especially important. While it is a well-known fact that internal temperatures follow those of the air and soil closely, though with varying rapidity of response, this holds less for leaves than for stems and roots. Owing to the very obvious difficulties, practically nothing has yet been done in this important field. A few preliminary results have been obtained at Minnehaha, which serve to show the need for such readings. Gravel slide rosettes in an air temperature of 24° C. and a surface temperature of 40° C. gave the following surface readings:Parmelia, 40°,Eriogonum, 38.6°,Arctostaphylus, 35°,Thlaspi, 31.8°, andSenecio, 31°. The leaf ofEriogonum flavum, which is smooth above and densely hairy below, indicated a temperature of 31.8° when rolled closely about the thermometer bulb with the smooth surface out, and 28° when the hairy surface was outside. The surface readingsof the same leaf were .5°–1° higher when made upon the upper smooth surface. This immediately suggests that the lower surface may be modified to protect the leaf from the great heat of the gravel, which often reaches 50° C. (122° F.).

105. General relations.As the factor which exerts the most important control upon water-content and humidity, rainfall must be carefully considered by the ecologist. It is such an obvious factor, and is usually spoken of in such general terms that the need of following it accurately is not evident at once. When it is recognized that the fluctuations of water-content are directly traceable to it, it becomes clear that its determination is as important as that of any indirect factor. This does not mean, however, that the amount of yearly rainfall is to be taken from the records of the nearest weather station, and the factor dismissed. Like other instruments, the rain gauge must be kept at the base station of the area under study, and when this is extensive or diverse, additional instruments should be put into commission. While the different parts of the same general climatic region may receive practically the same amount of precipitation during the year, it is not necessarily true that the rainfall of any particular storm is equally distributed, especially in the mountains. Nothing less than an exact knowledge of the amount of rain that falls in the different areas will make it possible to tell how much of the water-content found at any particular time in these represents merely the chance differences of precipitation.

The forms of precipitation are rain, dew, hail, snow, and frost. Of these, hail is too infrequent to be taken into account, while frost usually occurs only at the extremes of the growing season, and in its effect is rather to be reckoned with temperature. Snow rarely falls except during the period of rest, and, while it plays an important part as cover, it is merely one of several factors that determine the water-content of the soil at the beginning of spring. The influence of dew is not clearly understood. It is almost always too slight in amount and too fleeting to affect the water-content of the soil. It seems probable that it may serve by its own evaporation to decrease in some degree the water loss from the soil, and from bedewed plants. If, however, the dew is largely formed by the water of the soil and of the plant, as is thought by some, then it is negligible as a reinforcement of water-content. From the above, it is evident that rainfall alone exerts a profound effect upon the habitat, and it is with its measurement that the ecologist is chiefly concerned.

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