LIGHT
76. Methods. All methods for measuring light intensity, which have been at all satisfactory, are based upon the fact that silver salts blacken in the light. The first photographic method was proposed by Bunsen and Roscoe in 1862; this has been taken up by Wiesner and variously modified. After considerable experiment by the writer, however, it seemed desirable to abandon all methods which require the use of “normal paper” and “normal black” and to develop a simpler one. As space is lacking for a satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited below.[[4]] Simple photometers for making light readings simultaneously or in series were constructed in 1900, and have been in constant use since that time. An automatic instrument capable of making accurate continuous records proved to be a more difficult problem. A sunshine recorder was ultimately found which yields valuable results, and very recently a recording photometer which promises to be perfectly satisfactory has been devised. Since the hourly and daily variations of sunlight in the same habitat are relatively small, automatic photometers are perhaps a convenience rather than a necessity.
The Photometer
Fig. 11. Photometer, showing front and side view.
77. Construction. The simple form of photometer shown in the illustration is a light-tight metal box with a central wheel upon which a strip of photographic paper is fastened. This wheel is revolved by the thumbscrew past an opening 6 mm. square which is closed by means of a slide working closely between two flanges. At the edge of the opening, and beneath the slide is a hollow for the reception of a permanent light standard. The disk of the thumbscrew is graduated into twenty-five parts, and these are numbered. A line just beneath the opening coincides with the successive lines on the disk, and indicates the number of the exposure. The wheel contains twenty-five hollows in which the click works, thus moving each exposure just beyond the opening. The metal case is made in two parts, so that the bottom may be readily removed, and the photographic strip placed in position. The water-photometer is similar except that the opening is always covered with a transparent strip and the whole instrument is water-tight. These instruments have been made especially for measuring light by the C. H. Stoelting Co., 31 W. Randolph street, Chicago, Ill. The price is $5.
78. Filling the photometer. The photographic paper called “solio” which is made by the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for photometric readings. The most convenient size is that of the 8 × 10 inch sheet, which can be obtained at any supply house in packages of a dozen sheets for 60 cents. New “emulsions,” i. e., new lots of paper, are received by the dealers every week, but each emulsion can be preserved for three to six months without harm if kept in a cool, light-tight place. Furthermore, all emulsions are made in exactly the same way, and it has been impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm. wide is cut lengthwise from the 8 × 10 sheet. This must be done in the dark room, or at night in very weak light. The strip is placed on the wheel, extreme care being taken not to touch the coated surface, and fixed in position by forcing the free ends into the slit of the wheel by a piece of cork 8–9 mm. long. The wheel is replaced in the case, turned until the zero is opposite the index line, and the instrument is ready for use.
79. Making readings. An exposure is made by moving the slide quickly in such a way as to uncover the entire opening, and the standard if the exposure is to be very short. Care must be taken not to pull the slide entirely out of the groove, as it will be impossible to replace it with sufficient quickness. The time of exposure can be determined by any watch after a little practice. It is somewhat awkward for one person to manage the slide properly when his attention is fixed upon a second hand. This is obviated by having one observer handle the watch and another the photometer, but here the reaction time is a source of considerable error. The most satisfactory method is to use a stop-watch. This can be held in the left hand and started and stopped by the index finger. The photometer is held against it in the right hand in such a way that the two movements of stopping the watch and closing the slide may be made at the same instant. The length of exposure is that necessary to bring the tint of the paper to that of the standard beside it. A second method which is equally advantageous and sometimes preferable does away with the permanent standard in the field and the need for a stop-watch. In this event, the strip is exposed until a medium color is obtained, since very light or very deep prints are harder to match. This is later compared with the multiple standard. In both cases, the date, time of day, station, number of instrument and of exposure, and the length of the latter in seconds are carefully noted. The instrument is held with the edge toward the south at the level to be read, and the opening uppermost in the usual position of the leaf. When special readings are desired, as for isophotic leaves, reflected light, etc., the position is naturally changed to correspond. In practice, it is made an invariable rule to move the strip for the next exposure as soon as the slide is closed. Otherwise double exposures are liable to occur. When a strip is completely exposed it is removed in the dark, and a new one put in place. The former is carefully labeled and dated on the back, and put away in a light-tight box in a cool place.
Fig. 12. Dawson-Lander sun recorder.
80. The Dawson-Lander sun recorder. “The instrument consists of a small outer cylinder of copper which revolves with the sun, and through the side of which is cut a narrow slit to allow the sunshine to impinge on a strip of sensitive paper, wound round a drum which fits closely inside the outer cylinder, but is held by a pin so that it can not rotate. By means of a screw fixed to the lid of the outer cylinder, the drum holding the sensitive paper is made to travel endwise down the outer tube, one-eighth of an inch daily, so that a fresh portion of the sensitive surface is brought into position to receive the record.” The instrument is driven by an eight-day clock placed in the base below the drum. The slit is covered by means of a flattened funnel-shaped hood, and the photographic strip is protected from rain by a perfectly transparent sheet of celluloid. The detailed structure of the instrument is shown in figure 12. This instrument may be obtained from Lander and Smith, Canterbury, England, for $35.
In setting up the sunshine recorder, the axis should be placed in such a position that the angle which it makes with the base is the same as the altitude of the place where the observations are made. This is readily done by loosening the bolts at either side. The drum is removed, the celluloid sheet unwound by means of the key which holds it in place, the sensitive strip put in position, and the sheet again wound up. Strips of a special sensitive paper upon which the hours are indicated are furnished by the makers of the instrument, but it has been found preferable to use solio strips in order to facilitate comparison with the standards. The drum is placed on the axis, and is screwed up until it just escapes the collar at the top of the spiral. The clock is wound and started, and the outer cylinder put on so that the proper hour mark coincides with the index on the front of the base.
As a sunshine recorder, the instrument gives a perfect record, in which the varying intensities are readily recognizable. Since the cylinder moves one-half inch in an hour, and the slit is .01 of an inch, the time of each exposure is 72 seconds. This gives a very deep color on the solio paper, which results in a serious error in making comparisons with the standard. On account of the hood, diffuse light is not recorded when it is too weak to cast a distinct shadow. It seems probable that this difficulty will be overcome by the use of a flat disk containing the proper slit, and in this event the instrument will become of especial value for measuring the diffuse light of layered formations. The celluloid sheet constitutes a source of error in sunlight on account of the reflection which it causes. This can be prevented by using the instrument only on sunny days, when the protection of the sheet can be dispensed with.
81. The selagraph. This instrument is at present under construction, and can only be described in a general way. In principle it is a simple photometer operating automatically. It consists of a light-tight box preferably of metal, which contains an eight-day lever clock. Attached to the arbor of the latter is a disk 7 inches in diameter bearing on its circumference a solio strip 1 cm. wide and 59 cm. long. The opening in the box for exposure is 6 mm. square and is controlled by a photographic shutter. The latter is constructed so that it may be set for 5, 10, or 20 seconds, since a single period of exposure can not serve for both sun and shade. The shutter is tripped once every two hours, by means of a special wheel revolving once a day. Each exposure is 6 mm. square, and is separated by a small space from the next one. Twelve exposures are made every 24 hours, and 84 during the week, though, naturally, the daytime exposures alone are recorded. Comparisons with the multiple standard are made exactly as in the case of the simple photometer. The selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.
Standards
82. Use. The light value of each exposure is determined by reference to a standard. When the photometer carries a permanent standard, each exposure is brought to the tint of the latter, and its value is indicated by the time ratio between them. Thus, if the standard is the result of a 5–second exposure to full sunlight at meridian, and a reading which corresponds in color requires 100 seconds in the habitat concerned, the light of the latter is twenty times weaker or more diffuse. Usually, the standard is regarded as unity, and light values figured with reference to it, as .05. With the selagraph such a use of the standard is impossible, and often, also, with the photometer it is unnecessary or not desirable. The value of each exposure in such case is obtained by matching it with a multiple standard, after the entire strip has been exposed. The further steps are those already indicated. After the exact tint in the standard has been found, the length of the reading in seconds is divided by the time of the proper standard, and the result expressed as above.
83. Making a standard. Standards are obtained by exposing the photometer at meridian on a typically clear day, and in the field where there is the least dust and smoke. Exception to the latter may be made, of course, in obtaining standards for plant houses located in cities, though it is far better to have the same one for both field and control experiment. Usable standards can be obtained on any bright day at the base station. Indeed, valuable results are often secured by immediate successive sun and shade readings in adjacent habitats, where the sun reading series is the sole standard. Preferably, standards should be made at the solstices or equinoxes, and at a representative station. The June solstice is much to be preferred, as it represents the maximum light values of the year. Lincoln has been taken as the base station for the plains and mountains. It is desirable, however, that a national or international station be ultimately selected for this purpose, in order that light values taken in different parts of the world may be readily compared.
84. Kinds of standards. The base standard is the one taken at Lincoln (latitude 41° N.) at meridian June 20–22. This is properly the unit to which all exposures are referred, but it has been found convenient to employ the Minnehaha standard as the base for the Colorado mountains, in order to avoid reducing each time. Relative standards are frequently used for temporary purposes. Thus, in comparing the light intensities of a series of formations, one to five standards are exposed on the solio strip before beginning the series of readings. Proof standards are the exposed solio strips, which fade in the light, and can, in consequence, be kept only a few weeks without possibility of error. The fading can be prevented by “toning” the strip, but in this event the exposures must be fixed in like manner before they can be compared. This process is inconvenient and time-consuming. It is also open to considerable error, as the time of treatment, strength of solution, etc., must be exactly equivalent in all instances. Permanent standards are accurate water-color copies of the originals obtained by the photometer. These have the apparent disadvantage of requiring a double comparison or matching, but after a little practice it is possible to reproduce the solio tints so that the copy is practically indistinguishable from the original. The most satisfactory method is to make a long stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and then to reject such parts of the stroke as do not match exactly. Permanent standards fade after a few month’s use, and must be replaced by parts of the original stroke. Single standards are made by one exposure, while multiple ones have a series of exposures filling a whole light strip. These are regularly obtained by making the exposures from 1–10 seconds respectively, and then increasing the length of each successive exposure by 2 seconds. Single exposures of 1–5 seconds as desired usually serve as the basis for permanent standards, but a multiple standard may also be copied in permanent form. Exposures for securing standards must be made only under the most favorable conditions, and the length in seconds must be exact. The use of the stop-watch is imperative, except where access may be had to an astronomical clock with a large second hand, which is even more satisfactory. The length of time necessary for the series desired is reckoned beforehand, and the exposures begun so that the meridian falls in the middle of the process.
Single standards are exceedingly convenient in photometer readings, but they are open to one objection. In the sunshine it is necessary to make instant decision upon the accuracy of the match, or the exposure becomes too deep. In the shade where the action is slower, this difficulty is not felt. For this reason it is usually desirable to check the results by a multiple standard, and in the case of selagraph records, where the various exposures show a wide range of tint, light values are obtainable only by direct comparison with the multiple standard. The exact matching of exposure and standard requires great accuracy, but with a little practice this may be done with slight chance of error by merely moving the exposure along the various tints of the standard until the proper shade is found. The requisite skill is soon acquired by running over a strip of exposures several times until the comparisons always yield the same results for each. The margin of error is practically negligible when the same person makes all the comparisons, and in the case of two or three working on the same reading the results diverge little or not at all. The efficient difference for light is much more of a variable than is the case with water-content. It has been determined so far only for a few species, all of which seem to indicate that appreciable modification in the form or structure of a leaf does not occur until the reduction in intensity reaches .1 of the meridian sunlight at the June solstice. The error of comparison is far less than this, and consequently may be ignored, even in the most painstaking inquiry.
Readings
85. Time. The intensity of the light incident upon a habitat varies periodically with the hour and the day, and changes in accord with the changing conditions of the sky. The light variations on cloudy days can only be determined by the photometer. While these can not be ignored, proper comparisons can be instituted only between the readings taken on normal days of sunshine. The sunlight varies with the altitude of the sun, i. e., the angle which its rays make with the surface at a given latitude. This angle reaches a daily maximum at meridian. The yearly maximum falls on June 22, and the angle decreases in both directions through the year to a minimum on December 22. At equal distances from either solstice, the angle is the same, e. g., on March 21 and September 23. At Lincoln (41° N. latitude) the extremes at meridian are 73° and 26°; at Minnehaha (39°) they are 75° and 28°. The extremes for any latitude may be found by subtracting its distance in degrees north of the two tropics from 90. Thus, the 50th parallel is 26.5° north of the tropic of Cancer, and the maximum altitude of the sun at a place upon it is 63.5°. It is 73.5° north of the tropic of Capricorn, and the minimum meridional altitude is 16.5°.
The changes in the amount of light due to the altitude of the sun are produced by the earth’s atmosphere. The absorption of light rays is greatest near the horizon, where their pathway through the atmosphere is longest, and it is least at the zenith. The absorption, and, consequently, the relative intensity of sunlight, can be determined at a given place for each hour of any sunshiny day by the use of chart 13. This chart has been constructed for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The curves which show the altitude of the sun at the various times of the day and the year have been constructed by measurements upon the celestial globe. Each interval between the horizontal lines represents 2 degrees of the sun’s altitude. The vertical lines indicate time before or after the apparent noon, the intervals corresponding to 10 minutes. If the relative intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent noon for this day must first be determined. A glance at the table shows that the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The apparent noon at Lincoln is found by adding 26 minutes 49 seconds, the difference in time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23 minutes later. The sun’s altitude is accordingly 36°. If the intensity of the light which reaches the earth’s surface when the sun is at zenith is taken as 1, the table of the sun’s altitudes gives the intensity at 3:00 P.M. on March 12 as .85.
For places with a latitude differing by several degrees from that of Lincoln, it is necessary to construct a new table of altitude curves from the celestial globe. It is quite possible to make a close approximation of this from the table given, since the maximum and minimum meridional altitude, and hence the corresponding light intensity, can be obtained as indicated above. For Minnehaha, which is on the 105th meridian, and for other places on standard meridians, i. e., 60°, 75°, 90°, and 120° W., the table of apparent noon indicates the number of minutes to be added to 12 noon, standard time, when the sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of a standard meridian is respectively faster or slower than the latter. The exact difference in minutes is obtained from the difference in longitude by the equation, 15° = 1 hour. Thus, Lincoln, 96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is consequently 26 minutes 49 seconds slower, and this time must always be added to the apparent noon as determined from the chart. At a place east of a standard meridian, the time difference is, of course, subtracted.
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 6:00 A.M. and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.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 8:00 A.M. and 4:00 P.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 8:00 A.M. and 4:00 P.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 astronomical method 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
| DATE | TIME EQUATION | LINCOLN NOON | |||
|---|---|---|---|---|---|
| Sun slow: + | 26m. | 49s. | |||
| January | 1 | 3m. | 47s. | 12:31 | P.M. |
| „ | 6 | 6 | 7 | :33 | |
| „ | 11 | 8 | 12 | :35 | |
| „ | 16 | 10 | 3 | :37 | |
| „ | 21 | 11 | 35 | :38 | |
| „ | 26 | 12 | 48 | :40 | |
| „ | 31 | 13 | 41 | :40 | |
| February | 10 | 14 | 27 | :41 | |
| „ | 20 | 13 | 56 | :41 | |
| March | 2 | 12 | 18 | :39 | |
| „ | 7 | 11 | 10 | :38 | |
| „ | 12 | 9 | 53 | :37 | |
| „ | 17 | 8 | 29 | :36 | |
| „ | 22 | 6 | 59 | :34 | |
| „ | 27 | 5 | 27 | :32 | |
| April | 1 | 3 | 55 | :31 | |
| „ | 6 | 2 | 27 | :29 | |
| „ | 11 | 1 | 3 | :28 | |
| Sun fast: − | |||||
| „ | 16 | 0 | 13 | :27 | |
| „ | 21 | 1 | 20 | :25 | |
| „ | 26 | 2 | 16 | :24 | |
| May | 1 | 3 | 0 | :24 | |
| „ | 16 | 3 | 48 | :23 | |
| „ | 31 | 2 | 33 | :24 | |
| June | 5 | 1 | 45 | :25 | |
| „ | 10 | 0 | 49 | :26 | |
| Sun slow: + | |||||
| „ | 15 | 0 | 13 | :27 | |
| „ | 20 | 1 | 18 | :28 | |
| „ | 25 | 2 | 22 | :29 | |
| „ | 30 | 3 | 22 | :30 | |
| Sun slow: + | 26m. | 49s. | |||
| July | 5 | 4m. | 19s. | 12:31 | P.M. |
| „ | 10 | 5 | 7 | :32 | |
| „ | 20 | 6 | 6 | :33 | |
| August | 4 | 5 | 53 | :33 | |
| „ | 14 | 4 | 30 | :31 | |
| „ | 19 | 3 | 28 | :30 | |
| „ | 24 | 2 | 13 | :29 | |
| „ | 29 | 0 | 48 | :28 | |
| Sun fast: − | |||||
| September | 3 | 0 | 45 | :26 | |
| „ | 8 | 2 | 25 | :24 | |
| „ | 13 | 4 | 9 | :23 | |
| „ | 18 | 5 | 55 | :21 | |
| „ | 23 | 7 | 41 | :19 | |
| „ | 28 | 9 | 23 | :17 | |
| October | 3 | 10 | 59 | :16 | |
| „ | 8 | 12 | 26 | :14 | |
| „ | 13 | 13 | 43 | :13 | |
| „ | 18 | 14 | 48 | :12 | |
| „ | 23 | 15 | 37 | :11 | |
| November | 2 | 16 | 20 | :10 | |
| „ | 12 | 15 | 45 | :11 | |
| „ | 17 | 14 | 54 | :12 | |
| „ | 22 | 13 | 44 | :13 | |
| „ | 27 | 12 | 14 | :15 | |
| December | 2 | 10 | 25 | :16 | |
| „ | 7 | 8 | 21 | :18 | |
| „ | 12 | 6 | 5 | :21 | |
| „ | 17 | 3 | 41 | :23 | |
| „ | 22 | 1 | 12 | :26 | |
| Sun slow: + | |||||
| „ | 27 | 1 | 17 | :28 | |
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 page [57] will 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 8:00 A.M. and 4:00 P.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 in Elodea is 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 the sun, 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.
Table of Intensity at Various Angles
| ANGLE | INTENSITY |
|---|---|
| 90 | 1.00 |
| 80 | .98 |
| 70 | .94 |
| 60 | .87 |
| 50 | .77 |
| 40 | .64 |
| 30 | .50 |
| 20 | .34 |
| 10 | .17 |
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.
Reflected and Absorbed Light
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 amount received, 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., 11 A.M. August 20. The leaves are from sun and shade forms of Bursa bursa-pastoris, Rosa sayii, Thalictrum sparsiflorum, and Machaeranthera 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, or epidermis prints, are referred to a multiple standard. 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 of Achillea lanulosa, Capnoides aureum, Antennaria umbrinella, Galium boreale, and Potentilla 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 from leaves 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.
Expression of Results
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:
| Day | Hour | Formation | Station | Altitude | Exposure | Group | Height | No. | Length of exposure | Standard | Light value | Base value | Reflected light | Transm’d light | Absorbed light |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 14/9/04 | 12:00 M. | Spruce | Milky Way | 2600 m. | N.E. 20° | Opulaster | 1 foot | 2:10 | 160 s. | 3 s | .019 | ||||
| „ | 12:05 P.M. | Spruce | Moss Glen | 2500 m. | Level | Streptopus | „ | 2:12 | 240 s. | 3 s. | .012 | ||||
| „ | 12:15 P.M. | Brook b’nk | Grotto | 2500 m. | E. 3° | Filix | Surface | 2:13 | 360 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 of daily duration, based upon full sunlight, is also of especial importance for plants, and stations which receive both sun and shade during the day.