SECOND REPLY. 159 



before we shall discover all the separate links, which shall 

 perfectly complete the chain. The theory of the interchange 

 of matter tells us only in general terms, what goes on between 

 plants and animals, animals and atmosphere, atmosphere 

 and plants, but says not what processes occur in the plant, 

 in the animal, but it indeed restrains our further investiga- 

 tions so far, that we must henceforth reject every illus- 

 trative experiment as false, which contradicts that theory of 

 the interchange. All experiments, for instance, for the 

 purpose of deducing the nutrition of plants from the organic 

 constituents of the soil, are at once rendered vain, because 

 we know certainly from that theory, that we can never 

 account even for the fourth part of the plants growing 

 upon the soil, from the whole of the organic matter con- 

 tained in it. 



But an objection here starts up, quite independently, 

 which appears very unfavourable to the whole theory. We 

 most undoubtedly see that cultivated plants thrive better in 

 soils rich in humus or well manured fields, than in those 

 not manured. If then the plant draws carbonic acid, 

 ammonia and water from the air, if this is its only source 

 of food, what is the use of the manure ? Why must we 

 use it, if we would not renounce that flourishing condition 

 of the plants under culture ? This question can only be 

 met by two answers, one derived from physics, the 

 other from chemistry ; the one explaining the action of 

 humus in general, the other the especial necessity for, and 

 advantage of, the use of manure. 



Carbonic acid, ammonia and watery vapour are cer- 

 tainly the food of plants, but the question is, by what 

 organs do they absorb these matters? In regard to 

 water, there is no doubt it must be wholly, or up to 99 

 per cent, absorbed by the roots. From the experiments 



