Evolution of Structure in Seedlings. 51 



evolution of structure is independent of light, and tliat the 

 manner of growth during the day is similar to that at night. 



It will be noticed that the total average height attained in 

 the light is only about half that in the dark series. The ex- 

 planation of this we have already seen in the fact that in the 

 former the leaves and tendrils were much larger than in the 

 latter, while the dry weights were nearly the same. The 

 material of the seed in the light series was consumed in ex- 

 tending these surfaces, while in the dark series it was spent in 

 lengthening the stem. 



Having established the continuous chai-acter of growth in 

 seedlings, and the similarity of rate and nature of the process 

 by night and by day, and admitting that night plants throw 

 off carbonic acid, it is not improbable that this carbonic acid 

 arises, not from mechanical absorption by the roots and vapori- 

 zation by the leaves, but as a direct result or concomitant of 

 the act or process of evolution of structure. 



To put the matter in the clearest form, let us first under- 

 stand what growth is. It appears in all cases to consist in the 

 evolution or production of cells from those already existing. 

 According as the circumstances under which the cells are pro- 

 duced vary, so does the tissue ultimately produced vary ; cells 

 formed in woody fibre become wood ; cells formed in muscle 

 in their turn form muscles ; but the starting-point of the process 

 in every instance is the formation of new cells. 



If, now, we examine the evolution of cells under the simplest 

 conditions, as, for example, in the fermentation that attends tlie 

 manufacture of alcohol, we find that with the evolution of the 

 Torula-CQW^s, carbonic acid is produced. The two results are 

 intimately connected ; and it is proper to suppose that since the 

 carbonic acid has arisen along with the new cells, the latter 

 operation must in some way involve a process of oxidation. 

 Accepting the hypothesis that oxidation is attendant on these 

 processes of cell- growth under the simplest conditions, we pass 

 to the examination of what occurs in the lower forms of veget- 

 able organisms found in the air. 



The fungi, and, indeed, all plants that are not green, witli a 

 few exceptions, exhale carbonic acid and never exhale oxygen. 

 In this case, in which cell-production often occurs with such 

 marvellous rapidity, the carbonic acid must have arisen as a 

 consequent of tlie cell-growth. It is improbable that it lias 

 been absorbed by roots and exhaled from the structures, either 

 in these plants or in those produced during fermentation. In 

 the latter there never are any roots ; and in the former, even 

 where roots arc present, they bear a small proportion to the 

 whole plant. The quantity of moisture exhaled by such 



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