CHAPTER IX 



MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 



. The essential elements discussed in the previous section are used 

 ultimately in the construction of the plant's substance. They are indis- 

 pensible because the plant cannot be constructed unless each one of 

 them is present. 



ASSIMILATION AND LIMITING FACTORS DEFINED 



The term assimilation, in its broadest sense, is used to describe the 

 process by which a plant builds up the substances that comprise it out of 

 compounds obtained from its environment. To be sure any compound 

 will not serve; certain specific materials are necessary. Assimilation 

 depends on a supply of such materials and on a source of energy. The 

 amount of assimilation and hence of growth is determined by the opera- 

 tion of the principle of limiting factors. 



Most plants require at least seven elements in combined form from 

 the soil, namely, S, P, N, K, Fe, Mg and Ca. If «S, )8P, 7N, 5K, 

 eFe, rMg and 7?Ca combine exactly to produce a unit amount of growth 

 in some particular plant, say an apple tree, and if aS, 6P, cN, dK, eFe, 

 /Mg and gCa are present in a particular soil in available form, the maxi- 

 mum amount of apple tree tissue that can be grown in that soil will be 

 the smallest of the fractions a/a, h/^, c/y, d/8, e/e, //f, g/rj. That 

 element which gives the smallest fraction is the limiting factor of growth.®^ 



The principle of limiting factors applies not merely to nitrogen and the 

 essential mineral elements, but also to water, to carbon dioxide and to 

 oxygen which likewise are essential nutrients entering into the composi- 

 tion of the plant. Moreover the principle covers the effects of external 

 factors such as temperature and light which also may be limiting factors 

 of assimilation. All of these possible limiting factors of assimilation 

 and growth constitute the external stimuli to which the organism reacts 

 and these reactions tend to overcome the limiting factors of assimilation 

 and so bring the organism in the most favorable situation for assimila- 

 tion that circumstances permit. In consequence of the reactivity of 

 the plant and its apparent complete adjustment to its environment 

 the principle of limiting factors sometimes may seem not to be operative. 

 This, however, is not the case, for the principle of limiting factors is 

 always effective. The principle is generally recognized in the saying 

 that a chain is^no strenger than its weakest link and it is so universally 

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