CONDITIONS AFFECTING THE EXPERIMENTS. 295 



adapted to just this intensity of light, and but few species can flourish in the 

 deep shade of woods. In a word, the experimental plants, to which we have 

 hitherto confined our attention, can only yield a satisfactory result when artificially 

 nourished, if they are enabled, by being strongly illuminated, actually to make use of 

 the carbon dioxide present in the atmosphere. Without the co-operation of light of 

 sufficient intensity, the carbon dioxide of the atmosphere is as good as non-existent, 

 so far as the nutrition of the plant is concerned. The light, so to speak, affords to 

 the cells containing chlorophyll the forces necessary for separating the carbon of the 

 carbon dioxide from the oxygen, and uniting it at the same time to other elements, 

 and transforming it into organic plant-substance. Hence it has been said, and 

 correctly, that it is the sun which maintains vegetable life on the surface of the earth ; 

 and since the entire animal world derives its food from plants, it may be said that 

 it is the energy contained in the sun's rays which developes itself in the vital 

 movements of all terrestrial organisms — animals as well as plants. The mechanical 

 force contained in the solar rays acts in the chlorophyll-containing cells of the plant 

 like the movements of a watch-key, by which the spring of the clockwork is wound 

 up, and the vital processes of animals and plants resemble the slow running down 

 of the clockwork ; for by these vital processes the organic substance produced in the 

 chlorophyll-containing cells by means of the light is gradually destroyed again, and 

 finally re-transformed into carbon dioxide and water, until, on the commencement of 

 a new period of vegetation, the clockwork is wound up anew. 



Referring once more to our experiment on vegetation, after these comprehen- 

 sive considerations suggested by the results, from another side, we have still to 

 consider whether the experimental plants, nourished in an aqueous solution, grow 

 as normally and vigorously as if we had brought them up in good vegetable 

 soil. The experiment is easily arranged : it suffices to cause several seeds, 

 of the same species as those employed for the water-culture, to germinate and 

 develope in vessels containing an equal volume of good vegetable soil, and to 

 expose these near our experimental plants, and to the same light. It is found 

 by this that the vegetation is in both cases approximately equal, but in neither 

 case so vigorous as when the plants are cultivated in the ordinary manner in the 

 garden. They there meet not only with more direct light, and other advantages 

 which growth in the open affords, but also, and above all, the roots are able to 

 develope to more advantage in the open soil, becoming much longer and much more 

 copiously branched than in our water-culture; moreover they are in contact partly with 

 air, which they respire, and only partly with fluid, and their root-hairs attach them- 

 selves, as we already know, to the particles of soil, from which they take up absorbed 

 food-materials. This is simply the normal condition of true land-plants ; and their 

 stay in a nutritive solution must necessarily affect their function abnormally. 

 Nevertheless, the results of our water-culture are scientifically of value ; since they 

 show that although the roots are compelled to take up the food materials under 

 abnormal conditions, nevertheless with their help much vegetable substance is formed, 

 which behaves normally so far that with its aid the whole process of development 

 of a plant may be completed, including the formation of seeds capable of germination. 



