SYMBIOSIS AND METABIOS1S 243 



carbon-dioxide, supplying carbohydrate to the fungus. It is possible that 

 we have here a case of true parasitism. Not every parasite treats its host 

 plant so harshly as to kill it either wholly or partially ; the craftiness of the 

 parasite lies in this, that it keeps its attack within bounds such that the life 

 of the host is not imperilled, permitting in this way a longer period of useful- 

 ness and advantage on the part of the host. Reference should be made here 

 to the Uredineae and the Peronosporeae. In the case of lichens, however, 

 one sees not only that no damage is done to the algal cells, but that on the 

 contrary the fungus seems to stimulate them to further development ; the 

 algal cells are larger in the lichen than they are in the free condition. Still, 

 that fact does not negative the idea of parasitism, for in other cases we have 

 seen that an increase in size of the host-cells takes place owing to the stimulat- 

 ing effect of parasitic Fungi ; and we are able to explain the effect in some 

 measure by remembering that well-known poisons administered in small doses 

 are capable of acting as developmental stimuli. 



It is not within our power to enter into the numerous cases which have 

 with greater or less justice received the name of symbiosis. We may, however, 

 conclude that the term symbiosis may be applied in a more extended sense. 

 Symbiosis in the most restricted sense may be applied to a single organism formed 

 by the coming together of two symbionts, the combination possessing certain 

 constant functional peculiarities, which the constituents do not exhibit or exhibit 

 in a less degree (lichens, Leguminosae) ; on the other hand, to a certain extent, 

 we may speak of symbiosis when both symbionts are united only in so far 

 as, e. g. Clostridium pasteurianum is with the two aerobic Bacteria, in which 

 case they form an amorphous zoogloea ; and we may still speak of symbiosis 

 when green Algae and Clostridium act in conjunction with each other in arable 

 soil, and assist each other with their metabolic products. If we go a step 

 further, we meet with organisms which appear in the same situation, but follow- 

 ing each other, one preparing the soil for the other ; this phenomenon we may 

 term metabiosis (WARD, 1899). The close relationship between symbiosis and 

 metabiosis is obvious. We have already drawn attention to the wide distribu- 

 tion of metabiosis, and we may best express our views as to plant metabolism by 

 arranging in tabular form a review of the metabiosis of the various organisms 

 of which we have spoken. We shall limit ourselves in general to the two 

 elements which have played the chief part in our discussion, to the study, 

 that is to say, of the circulation of carbon and nitrogen in the organic world. 

 Chemistry tells us that no substance on our planet is ever lost ; but that an 

 active circulation of matter takes place on the earth, a circulation which is 

 closely connected with metabiosis. Every organism standing by itself would 

 soon have caused alterations in the outer world, which would have rendered 

 continued existence impossible. It is only by the existence of numerous 

 organisms with diversified functions that the perpetual renewal of life on 

 the globe is possible. 



The following table makes no claim to completeness ; even what it does 

 show is not perfect, since there are many metabolic products, which, if indi- 

 cated by arrows, would detract from the clearness of the diagram. The 

 carbon-assimilation of green plants is indicated in the middle of the table, 

 and for this the sunlight provides the requisite energy. On the storage of 

 solar energy in the carbohydrates are indirectly based all the processes which 

 are referred to in the summary. In the separate metabolic processes only 

 those products of metabolism which are of especial interest at the moment 

 are indicated. Chemical equations are omitted ; the outgoing and the final 

 products of a process are indicated by arrows which give also the direction 

 of the course of the reaction. If an expenditure of energy is necessary for 

 the reaction, the arrow is directed upwards ; a gain in energy is shown by 

 a downwardly directed arrow ; when the arrow is horizontally placed, it indi- 



R 2 



