290 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



These numbers may be regarded as the maxima and minima of relative light 

 requirement for these plants. The relative minimum increases with the geo- 

 graphical latitude. Acer platanoides, for example, has a relative minimum 

 light requirement of }{$ (0.018) at Vienna, %S (°-°3°) at Hamar, Norway, and 

 H (0.200) at Tromso, Norway. Of course the light intensity in the open 

 decreases with latitude, which suggests an explanation of this decrease in the 

 relative minimum light requirement. Also, with lower temperatures the mini- 

 mum light requirement is higher. 



There is also a relation between the light income of plants and mycorhiza, 

 the development of which occurs only in connection with plants that are con- 

 fined to shady situations. Finally, the amount of chlorophyll in plants and 

 the color of their leaves is related to the light income. 



In one of his later papers Wiesner 1 expresses the two following conclusions: 

 (1) Plants that are especially well adapted for growing in diffuse light are char- 

 acterized by having their green parts (especially their leaves, which are strongly 

 absorptive of light) so arranged as to receive light very freely; in many cases, 

 indeed, the leaves are so placed as to receive the maximum intensity of diffuse 

 light that the habitat affords. (2) Plants that are especially well adapted for 

 growing in direct sunshine, on the other hand, are characterized by leaves and 

 other green parts so placed as not to receive the light at its highest intensities, 

 but to receive only the lower intensities.* 



1 Wiesner, J., Ueber die Anpassung der Pflanze an das diffuse Tages- und das directe Sonnenlicht. Ann. 

 Jard. Bot. Buitenzorg. Supplement 3': 48-60. 1910. 



k In connection with the interpretation of all this work of Wiesner's it must be borne in 

 mind that his measurements were made in terms of the effect produced, by the radiation stud- 

 ied, upon photographic paper. This paper is especially sensitive to light radiation of the shorter 

 wave-lengths (blue-violet and ultra-violet), it is less sensitive to the medium wave-lengths of 

 light (green, yellow) and is almost wholly unaffected by the long wave-lengths of light 

 radiation (orange, red and infra-red). It is thus seen that the Weisner method automatically 

 applies a relative weighting to the effect produced by each one of the different wave-lengths 

 that constitute the radiant energy impinging upon the instrument, and the value obtained from 

 any test is the integration of these weighted partial values. The relative sensitivities of the 

 paper used might be experimentally determined for a variety of different short ranges of wave- 

 lengths, and weighting coefficients might thus be determined for each range, by the use of which 

 it might be possible to calculate from any given reading an approximate relative value for the 

 actual radiation intensity as a whole, providing all the tests dealt with radiation made up of 

 intensities of the various wave-lengths in a constant set of proportions. But the radiation to be 

 studied varies from place to place and from time to time, not only in total energy content, but 

 also in the relative proportions of the intensities of the various component wave-lengths; that 

 is, in quality. From these considerations it becomes evident that the Wiesner method, for 

 measuring and comparing the amounts of radiation received by plants in different places and 

 at different times, must be regarded as crude and unsatisfactory, at its very best. 



Besides this very serious physical objection to the method employing photographic paper, 

 there must be considered another objection that is just as serious, based on physiological rela- 

 tions. For the purposes of ecology and physiology it is necessary, not only that the quality 

 and intensity of the radiation received by plants in different places, etc., be measured and com- 

 pared as such, but that the physical values obtained by such measurement be subjected to a 

 physiological weighting, so as to give an index of the radiation received in each of the different 

 habitats as it may affect plants growing therein. It is unnecessary to add that the sensitive- 



