LIGHT IN RELATION TO TREE GROWTH. igt 



of birch (a light-demanding species) and of beech (shade- 

 enduring) were exposed to Hght of various intensities and 

 tested for the presence of starch. At a Hght intensity of i/6 

 of total daylight the leaves of beech showed distinctly the 

 presence of starch, while birch gave a less distinct reaction. 

 At a light intensity of i/io beech still gave a pronounced 

 reaction for starch, while in birch only traces of starch could 

 be detected. At a light intensity of i 50 birch leaves showed 

 no presence of starch, while the leaves of beech still continued 

 to form it vigorously. These facts tend to show the same point 

 brought out by Lubimenko, that there is a distinct difference in 

 the sensitiveness of the chloroplasts of beech and birch. 



Factors i/ifluencing Tolerance. — Tolerance varies not only with 

 species, but even within the same species, according to the 

 conditions under which the tree is growing. These variations 

 are due largely to changes in the structure of the leaves brought 

 about by changes in transpiration. Among the important 

 factors influencing transpiration, and therefore tolerance, are 

 climate, altitude, moisture and nourishment in the soil, age ot 

 tree, and vigour and origin of the individual. 



1. Climate. — Plants need less light the higher the temperature, 

 and more light the lower the temperature. Consequently the 

 higher the temperature of a given locality, the more shade a 

 tree can stand. This explains, in part, the frequent differences 

 of opinion regarding the tolerance of the same species when 

 the observations are taken in diflferent regions. For example. 

 White pine in Maine is less tolerant than in the Southern 

 Appalachians. In Vienna the minimum light intensity in which 

 the leaves of Norway maple {Acer platanoides) can exist is 155 

 of the total daylight. In Hamar, Norway, it is 1/37 ; in 

 Drottningholm, i 28; and in Tromso, 1/5. 



This substitution of light for heat in plant growth, and vice 

 versa, was well demonstrated by Wiesner (1907) with regard to 

 annual meadow grass {Poa annua). The minimum light 

 necessary for annual meadow grass at the beginning of March, 

 at Cairo, is equivalent to about 53 calories, while at Vienna it 

 is equivalent to 109. The mean temperatures are 59*9° and 

 35"6° F., respectively. 



2. Altitude. — The light requirement of a species increases also 

 with increase in altitude, but only to a certain limit, beyond 

 which it remains constant or even decreases. For example, 



