METHODS OF DETERMINING TOLERANCE. 47 
In a stand of very healthy and fast-growing saplings of Engel- 
mann spruce (Picea engelmannt), in small groups on the top of a 
ridge, under Engelmann spruce, he obtained the following values: 
0.050, 0.062, and 0.038. 
Arranging the species in the order of their light requirements, as 
determined by these observations, with the most intolerant first, gives 
the following scale: 
Western yellow pine. 
Douglas fir. 
White fir and Engelmann spruce. 
On the Wallowa Nationa! Forest, in northeastern Oregon, the 
light readings were not taken in so systematic a manner as at the 
Coconino Experiment Station, but in most cases three or more light 
readings were taken in each situation. The values given for each 
species are the average of all the readings taken at a number of 
situations: 
TABLE 6.—Light intensities, Wallowa National Forest. 
Light intensity—con- 
Number dition of saplings— 
Species. Elevation. of 
readings. 
Good. Poor. 
Feet. 
INOICSHIASTO CARD Aira sre ceyeno mie sce eerie Sera eleeeersens 7, 000-7, 500 12 0. 029 0. 006 
RICA EM elma mMM ey ME eee ee ya sleet ea 7, 000-7, 500 24 .021 . 004 
ANSE lnginecO OMe) Sec ccccosossouSSasbonocsecuobese 6, 000 3 OZ eal ertaais eto 
Abies grandis......-. PLFA De Ta RUED Sy st Re Ua aa ae BA 5, 000-6, 000 12 025 O11 
IRSeWGOtsusatarxal Ola erry yey le sae aL yA craven Oia 6, 000-7, 000 17 . 052 . 025 
Pinus contorta........ BN a A MAND Ee ARES ples ams ey Se 6, 000-7, 000 31 . 086 019 
WarixOccidembalisinygea yaks eee ee le eye ae 6,500 fs clas Mirae ed . 308 
The species are arranged according to their tolerance as deter- 
mined by experience, with the most tolerant first. It will be seen 
that the recorded light values roughly bear out this arrangement. 
DEFECTS OF THE METHOD. 
The inaccuracies of the photochemical method of measuring light 
intensity are not great. Ordinarily the error does not amount to more 
than 10 per cent, and by repeated measurements it can be reduced to 
5, and even to 2.5 per cent. 
A more serious objection to the method is that it measures only the 
chemical rays of the solar spectrum—that is, the rays of high refrangi- 
bility in the blue and violet end of the spectrum. It is assumed, how- 
ever, that the intensity of the red and yellow rays is proportional to 
the intensity of the chemical rays, and that measurement of the 
chemical rays therefore gives at the same time an approximate meas- 
ure of all other rays of the solar spectrum which affect plant growth. 
