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.
TEMPERATURE
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.
Thermometers
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 in gravel 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.
