THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 103 



former obtain all their nutriment directly from the inorganic world, and, so far 

 as their nutrition is concerned, are thus entirely independent of other organisms; 

 such forms we may speak of as autotrophic ; the other we designate hetero- 

 trophic^ and without the aid of autotrophic organisms their existence is im- 

 possible. From this it will be at once apparent how important a part is played 

 in nature by autotrophic plants, and for that reason they claim the first place 

 in our attention. 



Organic compounds of carbon are not available for the support of plants 

 grown in typical water-culture solutions, because the air contains none, or, at all 

 events, quantities so minute that their presence may be entirely neglected, and 

 because the nutritive solution in which the roots are immersed consists of inorganic 

 salts only. Since maize, buckwheat, and many other green plants thrive just as 

 well in such water-cultures as in the soil, it follows that the organic materials 

 occurring in ordinary soils are either not absorbed at all or that they are at least 

 unessential. We are thus driven, by the exclusion of other possibilities, to believe 

 that the source of carbon to the green plant must be the carbon-dioxide of the air. 

 This gas always occurs, although only in relatively small quantities, in the 

 atmosphere, and is never absent from the water of lakes, rivers, &c. Carbon- 

 dioxide also occurs dissolved in the water of a nutritive solution, unless special 

 precautions are taken for its exclusion, while in the soil it is generally present 

 abundantly. It is impossible to affirm right off whether a land plant obtains its 

 carbon-dioxide from the air by means of the leaves or from the soil by means of 

 the root. Experiment alone can settle this question ; and experiment clearly 

 teaches us that land plants cannot grow at all when they are prevented from 

 obtaining carbon-dioxide from the air ; at all events the carbon-dioxide absorbed 

 by means of their roots is insufficient. Again, it may be easily proved that 

 submerged plants are able to absorb all the carbon-dioxide they require by 

 means of the surface of their leaves from that naturally dissolved in the water. 



The fundamental thesis, therefore, we have to prove with regard to the 

 assimilation of carbon by autotrophic plants is this : the carbon-dioxide is 

 decomposed by the energy of sunlight acting on the chlorophyll bodies of the living 

 cells ; the carbon is united with the elements of water to form carbohydrates while 

 the oxygen is given off and escapes from the plant. 



[Following Pfeffer (Phys. ist and 2nd eds.) and Wiesner (Elemente 

 der Botanik), the production of organic material as well as its further altera- 

 tion into the complex constituents of the living cell, are described in this book 

 under the term assimilation. Objection has been taken to this nomenclature 

 in a review of this work in the Botanical Gazette (1904, 37, 390) (see also Bot. 

 Centrbl. 76, 257). The reviewer points out, with justice, that beginners are 

 liable to get into difficulties if the formation of sugar out of carbon-dioxide in 

 autotrophic plants, and the further transformation of sugar in heterotrophic 

 plants, are both described by the same name. Nevertheless, there appears to 

 us no good reason for accepting the alternative suggestion, viz. to describe 

 as synthesis or syntax, the assimilation of carbon and so distinguish it from 

 assimilation proper, for how should we then designate nitrogen assimilation 

 or assimilation of minerals ? In these cases we have no distinguishable simple 

 bodies, corresponding to the sugar, which are assimilated, but rather potassium 

 nitrate, or even the free nitrogen of the air. No good reason can be adduced 

 for treating carbon differently from nitrogen, and therefore we have retained 

 the older terminology, though with full cognizance of the difficulties involved 

 in so doing.] 



In order to render intelhgible this extremely important thesis in plant 

 physiology it will be necessary for us to investigate closely (i) the decomposition 

 of the carbon-dioxide ; (2) the significance of chlorophyll ; (3) the importance 

 of sunlight in the process ; (4) the nature of the resulting products. 



