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and the factors act not only separately, but also interfere with 

 one another, so that all sorts of complications arise. Howewer it 

 will frequently be the case, that a single factor, such as light, 

 temperature, the want of one or other substance in the soil, 

 surpasses in importance all other factors, so that it is this factor, 

 that either alone or chiefly determines the intensity of the production 

 of matter (Lie big's law of minimum.) When such a limiting factor 

 acts, the estimation of the production of matter will be very 

 much easier. 



The production of dry matter may be calculated as a diffe- 

 rence between income and expense. The income is mainly com- 

 posed of the organic matter produced by the C0 2 assimilation, 

 the expense on the other hand can include a series of different 

 accounts, such as the loss of matter by the respiration of leaves 

 and stem, by the fall of leaves, by the production of seeds and 

 so on. Though the question, as we said, is much complicated, 

 it should be possible by studying intensively certain typical plants 

 to calculate the sums of these different accounts separately, so 

 that the following scheme can be made: 



Expense. 

 Loss of dry matter 



of the leaves. 



by respiration <[ of the stem 

 of the root 



by leaf fall 



by production of seeds 



Sum of loss. 



Income. 

 Gain of dry matter 

 by G0 2 assimilation.. 



ash 



Sum of income 

 Sum of loss . . . 



organic matter produced 



In other words it should be possible to make up a balance- 

 sheet for the plant examined. 



More exactly the estimations for any given day are to be 

 carried out in the following manner. 



At first the leaf area of the plant in question is determined. 

 Further it is necessary to estimate the weight of the stem and 

 root and the dry matter content of the plant. The dry matter 

 content is calculated for a leaf area of 50 cm 2 . 



Income. Besides varying according to the specific proper- 

 ties of the plant, the C0 2 assimilation is especially influenced by 

 light and temperature. At first we must therefore measure the 

 intensity of light and temperature during the several hours of a 

 day, so that we can construct a curve, indicating the rate of 

 change of these two factors during a day. After that the G0 2 

 assimilation of the plant should be measured in different inten- 



