METHODS OF DETERMINING TOLERANCE. 438 
MEASUREMENTS OF DIRECT AND DIFFUSED LIGHT, 
Plants depend chiefly for their growth and development on dif- 
fused light; yet for certain functions, such as leafing, flowering, and 
fruiting, direct sunlight is essential, and it is therefore desirable to 
determine the re- 
quirements of trees 
for direct and dif- 
fused light. By 
means of Wiesner’s 
insolator the inten- 
sity of direct and 
diffused hght may 
be measured sepa- 
rately in the man- 
ner suggested by 
Roscoe. The  ob- rf) 
+ aan nia iseiEAEa 
server faces the sun, Au” 
holding the insola- mye. 9.—Hourly light intensities within the crown of ailan- 
tor in 2, horizontal thus (Ailanthus glandulosa). Horizontal arrangement of 
leaves. 
ee 
position before him. 
The time required to obtain the shade of a given standard is recorded. 
He then turns 180° so that his back is toward the sun, and holding 
the insolater in the shadow of his body again observes the time 
required for the 
“normal paper” to 
reach the tint of 
the same standard. 
The reading taken 
facing the sun 
gives the intensity 
of the total hght— 
that is, the sum of 
the direct and dif- 
fused light. The 
sa one facing away 
Eu wie ing WON? ie A we SGN 7 8 from the sun gives 
copy pet sit only the diffused 
Fic. 10.—Hourly light intensities within the crown of black hight, since here the 
eee (Robinia pseudacacia). Vertical arrangement of direct light ig cut 
off by the body of 
the observer. The intensity of the direct light alone is then found 
by subtracting the value of the diffused light from that of the total 
light. 
SNTIEN SITY OF LIGHT 
4 
AM, 
