46 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
Light Measurements in Relation to Minimum Requirements. 
The tolerance of trees to shade may be determined in any one of 
four ways : 
1. By preparing empirical scales of tolerance, based on experience 
and long- continued observation of the relative shade-enduring quali- 
ties of various species when growing together. This method is obvi- 
ously very crude, and may be very misleading, since such a condi- 
tion as soil moisture may determine, as directly as does light, the 
relative positions of the species in any particular stand. The per- 
sistence of individual branches, or rate of pruning; the maximum 
density of stands composed mainly of any particular species; the 
ability of reproduction to thrive in shade; all these things may be 
considered in preparing empirical scales. 
2. A second method of determining the tolerance of trees is by 
study of the structure of the leaves. Having determined the normal 
relations of the tissues of protective and assimilative characters, 
leaves may be subjected to different degrees of shading. Those which 
adapt themselves most completely to a variety of light conditions 
are naturally those which will survive best if placed under trying 
conditions as regards lack of light. This method, however scientifi- 
cally it is executed, can not give us absolute comparisons, since the 
structures of leaves are so variable even under the same conditions 
that the exact degree of change of structure can not be determined. 
In other words, this may give indications, but not comparable statis- 
tical data. 
3. A third method of determining tolerance is the experimental 
method, which must, of course, be executed in the laboratory where 
all other conditions, as well as the supply of light, may be con- 
trolled. The primary object is the determination of the minimum 
amounts of light which will sustain life of the several species under 
consideration, when all other conditions (especially heat and soil 
moisture) are nearly optimum. It will be fairly apparent, how- 
ever, that high temperatures may reduce the light requirement, and 
low soil moisture may increase it; and, since variations in all of the 
other conditions will be encountered in the field, it is very desirable 
that any experimental test should be so conducted that the influence 
of these other conditions on tolerance may be at least accurately 
gauged, if not directly measured. 
It is believed that the best results will be secured if each species 
to be tested is grown under a variety of light conditions, approach- 
ing both the optimum and the minimum, and if the tests are so 
conducted that the physiological effects of each light intensity may 
be expressed finally in terms of growth, or weight accretion, rather 
than if dependence is placed solely or largely on observations of 
