RELATIVE VALUE OF DIFFERENT METHODS. 53 
form, yet they are not the absolute quantities of light used by dif- 
ferent species. 
An apparent drawback to all the instruments used for measuring 
light directly is that they record only the light intensity at the given 
moment. The effect of light upon plants depends not only on its 
intensity but also on its duration during the day and during the 
entire vegetative period. The ideal method, therefore, of measuring 
light directly would be by a self-registering photometer, which 
would record the light intensities in the forest during the entire 
day and for the entire vegetative period. Such self-recording 
photometers, which would give the minima and maxima as well as 
the sum of the light enjoyed by trees, would make it possible to 
determine accurately the light requirements of our forest trees. They 
are, however, not yet in existence, and the best that can be done at 
present is to select typical clear days, determine the time of occur- 
rence of the minimum and maximum illumination of the interior of 
the crown, and in this way obtain the relative minima and maxima 
of light endured by different species. In interpreting the results of 
the light readings, account must be taken also of other factors of tree 
growth, such as moisture content of the soil, age and vigor of the 
specimen, quality of the soil, and relative humidity. 
With these precautions and some experience, it is possible by 
means of the photochemical method to obtain results of great prac- 
tical value, and to determine, if not the absolute quantities of light 
required by different species, at least their relative hght demands. 
The method still seems to be the one quick and practical method for 
measuring the light requirements of forest trees and expressing them 
in an accurate and mathematical form, and it has been already 
effectively used in a number of studies here and abroad. Though 
it is not free from drawbacks, these are bound to be eliminated more 
and more as the method itself receives greater recognition on the part 
of foresters and is tested by repeated observations. 
The relative importance of light in the life of the tree has long 
been recognized, but too little understood. Only recently have 
methods been devised for measuring light intensities, and all are 
admittedly imperfect. It is not the aim of this bulletin to recom- 
mend any particular method or instrument for measuring light, but 
to stimulate interest in this important problem and to indicate the 
different lines along which future practice may be developed. 
