THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 123 



We have thus learned that stomata constitute an apparatus of the very 

 greatest importance for assimilative purposes, an apparatus whose functional 

 activity is dependent in many respects on external factors. This dependence 

 on the environment is still further increased by the fact that external factors 

 affect the development of stomata. It is sufficient to note that stomata are not 

 developed in darkness. The same is true of chlorophyll, whose first appearance 

 and whose functional activity later is in the highest degree dependent on external 

 conditions. Only in certain Algae (Schimper, 1885) and in germinating Coni- 

 ferae (Burgerstein, 1900) is chlorophyll known to develop in darkness, but 

 in all higher plants the chromatophores develop yellow colouring matter only, 

 which has no effect on carbon-dioxide. Moreover chlorophyll decomposes in 

 darkness, sometimes slowly, at other times quickly, in a word, both the origin 

 and continued existence of chlorophyll depend on light. Further, a short ex- 

 posure to light of feeble intensity is sufficient to induce a development of the 

 green colour in the chloroplast, although subsequently placed in darkness. 

 Again, greening is induced with varying rapidity, although, by all visible rays of 

 the spectrum, and not by light of any definite wave length (Reinke, 1893). 

 The temperature necessary for the formation of chlorophyll must not be too 

 low ; between 0° and 5° C, light is able to induce an increase only in the yellow 

 colouring-matter already present (Elfving, 1880), and even seedlings of Coni- 

 ferae become green in darkness, as a general rule, only if the temperature be 

 over 9° C. 



Even when the chloroplasts develop normally, they cease acting as assimi- 

 lative agents under conditions which scarcely affect many of the other func- 

 tions of the cell, and which, at all events, act injuriously on life only 

 after prolonged application. Thus the activity of the chloroplasts may 

 be inhibited by extremes of temperature, by high percentages of carbon- 

 dioxide, by anaesthetics and antipyretics, acids and alkalis [compare Pan- 

 TANELLi, 1904], and also by prolonged insolation or accumulation of the pro- 

 ducts of assimilation (Ewart, 1896). [According to Arno Muller (1904), the 

 accumulation of the products of assimilation in leaves containing sugar 

 retards the decomposition of carbon-dioxide.] At the same time respiration con- 

 tinues quietly, the chloroplasts appear unchanged and regain their powers of 

 assimilation some time after normal conditions are reinstated. The fact that 

 the chlorophyll persists unaltered during these interferences with its functions, 

 shows that it is the protoplasmic framework that is affected. Both the com- 

 ponent parts of the chloroplast must work in harmony if it is to carry out its 

 normal functions (compare p. 109). Temporary inactivity of the chloroplasts 

 may be unintentionally induced in the course of experiment — for example, it 

 may take place very easily in amputated leaves owing to the accumulation of 

 assimilation products — and in consequence, experiments with isolated leaves do 

 not furnish us with accurate estimates of the amount of carbohydrates arising 

 in the normal leaf (compare p. 114). 



We have as yet spoken only of certain external factors which because they 

 influence the stomata and chlorophyll, either as regards their structure or func- 

 tional activity, are also of importance in the assimilation process. We must 

 now consider external factors in so far as they affect assimilation directly, 

 but it is obvious that no hard and fast line can be drawn between such 

 external conditions as influence this process directly and those which influence 

 it indirectly. One and the same factor may act in both ways. In fact, carbon- 

 dioxide can, for example, when in greater concentration, inhibit the action 

 of the chlorophyll, and thus can induce a decrease in assimilative activity, 

 instead of causing the increase which we would expect on physical grounds. 

 Since we have already treated of the influence of carbon-dioxide on assimilation, 

 more especially in relation to its concentration, we have only to add here that 



