LIGHT IN RELATION TO TREE GROWTH. I 85 



that of the total daylight, while in the shade of the trees, even 

 when they were still without any foliage, the light intensity was 

 one-fourth that of the total daylight. 



The minimum intensity of light in which photosynthesis can 

 take place is not sufficiently determined for all species ; it differs 

 in different species with the sensitiveness of the chloroplasts. 

 Trees not only accumulate energy by building up new organic 

 substance, but they also expend energy from the organic 

 substance which they produce. This expenditure of energy is 

 accompanied by oxidation of carbon and exhalation of carbon 

 dioxide, or respiration. As long as the light intensity is above 

 the necessary minimum for the given species, the process of 

 assimilating carbon from the air, and thus building up new 

 organic substance, goes on with greater energy than the opposite 

 process of breaking up organic substance and giving off carbon 

 into the air in the form of carbon dioxide. As the light intensity 

 decreases, the assimilation decreases correspondingly, and the 

 amount of carbon assimilated from the air approaches the amount 

 given otf 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 centimetre of a 

 leaf of European hornbeam ( Carpinus betitliis) on a cloudy 

 day decomposed in one hour 137 cubic millimetres of carbon 

 dioxide,^ or seven times as much as was given off by the leaf 

 in breathing; with a looo-candlepower electric light (which has 

 an effect upon assimilation similar to that of sunlight), at a 

 distance of 31 centimetres (i2'2 inches), a square centimetre of 

 leaf-surface decomposed 2^*5 cubic millimetres of carbon 

 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 i to i -5 metres (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. 



^ Equivalent 10 i square inch of leaf-surface decomposing 0"0054 cubic incli 

 of carbon dioxide. 



■^ Equivalent to i square inch of leaf-surface decomposing o"oii2 cubic inch 

 of carbon dioxide. 



VOL. XXVI. PART. II N 



