LIGHT INTENSITIES AND TREE GROWTH. 9 
process of breaking up organic substance and giving off carbon into 
the air in the form of carbon dioxid. As the light intensity de- 
creases the assimilation decreases correspondingly, and the amount 
of carbon assimilated from the air approaches the amount given off 
by respiration. As soon as the energy of assimilation falls so low 
that the amount of carbon assimilated is less than that needed for 
the maintenance of respiration, the leaf dies. 
It is possible to gain an idea of the relation between light intensity 
and the activity of the green cell from the experiments made by 
Kreusler. He found that a square centimeter of a leaf of Kuropean 
hornbeam (Carpinus betulus) on a cloudy day decomposed in one 
hour 13.7 cubic millimeters of carbon dioxide, or seven times as much 
as was given off by the leaf in breathing; with a 1,000-candlepower 
electric light (which has an effect upon assimilation similar to that 
of sunlight), at a distance of 31 centimeters (12.2 inches), a square 
centimeter of leaf surface decomposed 28.5 cubic millimeters of car- 
bon dioxide,? an amount 15 times greater (by weight) than that which 
was given off; by removing the source of light to a distance of from 
1 to 1.5 meters (39 to 59 inches), thus making it from one-ninth to 
one twenty-fifth as intense as before, the amount of carbon dioxide 
which was absorbed was barely enough to cover the amount given off 
by respiration. : i 
According to Wiesner (1907), for Norway maple (Acer plata- 
noides) the minimum light intensity under which photosynthesis 
can still take place 1s 1/55 of the total daylight, for birch 1/50, and 
for beech somewhat less than for birch. The fact that in light 
intensity of 1/70 of the total daylight, such as prevails in dense 
spruce forests, no green vegetation occurs under the shade of the 
trees indicates that the minimum light intensity at which a green 
leaf can still function must le between 1/60 and 1/70 of the total 
daylight intensity. 
As far as woody species are concerned, Wiesner (1907) found in 
the shade of European hornbeam (Carpinus betulus), with a light 
intensity of 1/58, an undergrowth of the following species still in a 
fairly good condition: Beech (Fagus sylvatica), hornbeam (Carpi- 
nus betulus), field maple (Acer campestre), common elm (Udmus 
campestris), and dogwood (Cornus sanguinea). In the same light 
intensity, however, the undergrowth of elder (Sambucus nigra), 
Huonymus europeus, and EL’. verrucosus, was dying. 
The minimum light intensity at which green vegetation disappears 
from under the shade of trees in the forest varies considerably with 
the climate. Thus, in the Temperate Zone no green vegetation occurs 
1 Equivalent to 1 square inch of leaf surface decomposing 0.0054 cubic inch of carbon 
dioxide. 
2 Hquivalent to 1 square inch of leaf surface decomposing 0.0112 cubic inch of carbon 
dioxide. 
