52 LIGHT IN RELATION TO TREE GROWTH. 
open, and therefore the measurement of light by means of photo- 
graphic paper gives the measure of the entire spectrum as well as 
of the chemical rays. The reason for this he finds in the fact that 
under ordinary conditions the light in the forest instead of being 
transmitted directly through the leaves enters almost wholly by re- 
flection through openings in the crowns. Thus, according to his cal- 
culations, the open, illumined space within the crown of a Norway 
inaple (Acer platanoides) is 670 times and that of cottonwood (Pop- 
ulus deltoides) 1,000 times larger than the space occupied by the 
solid substances of the two respective crowns. Hence the amount of 
light which is transmitted through the green leaves is comparatively 
small as compared with the amount of unchanged light. which passes 
through the openings in the crowns. Spectroscopic observations by 
Wiesner showed no difference in the composition of the light inside 
and outside the forest, even at light intensities in the forest as low 
as one-eightieth of that in the open, where the absolute light intensity 
ranged from 0.5 to 1.0. 
Zederbauer’s experiments in themselves are not sufficiently accu- 
rate to be entirely conclusive. They were apparently made at dif- 
ferent times and therefore probably at various conditions of the 
atmosphere, and, with one exception, on cloudy or partly cloudy 
days. Furthermore, only the visible rays of the solar spectrum are 
measured by means of Zederbauer’s instrument. Consequently his 
method fails to measure all the rays of the solar spectrum which 
affect plant life, since this is influenced by the invisible as well as the 
visible rays. As a matter of fact, by the photochemical method a 
much larger portion of the solar spectrum is measured than by 
Zederbauer’s method. 
RELATIVE VALUE OF DIFFERENT METHODS. 
Since no practical instrument has so far been devised by which 
the intensity of the different rays of the solar spectrum can be meas- 
ured, the measurement of the chemical hght intensity remains for 
the present the nearest approach to the ideal. The photochemical 
method doubtless can be used to great advantage in open forests and 
forests of moderate density. To what extent the light in dense 
forests may be different in composition from that in the open is still 
debatable. If its composition differs as the critics of the photo- 
chemical method claim, then the measurement of chemical intensity 
alone gives too low values for the forest. If it does not, as Wiesner’s 
investigations apparently show, then the measurement of the chem- 
ical light intensity is correct, even in forests of great density. While 
the physical methods of measuring light in the forest are more direct 
and express the light requirements of forest trees in a mathematical 
