ASSIMILATION OF FOOD. 507 



The absorption of oxygen by plants at night, which has been con- 

 firmed it appears by experiment, is explained by Liebig as a process of 

 oxidation of the volatile oils. But this process must also go on in the 

 day, and therefore it fails as an explanation. 



The researches upon the absorption of nutriment in plants are of little 

 worth, because they have proceeded from prejudices, often of an oppos- 

 ing nature, without the least regard to the natural circumstances of ve- 

 getation. Land-plants do not grow in water, or in a soil saturated 

 with fluid. The moisture of the soil exists under peculiar circumstances, 

 upon the nature of which there is an absence of all investigation. It is 

 absorbed by solid substances, and can only be retained by some essential 

 modification of the process of absorption. There is no lack of bad ex- 

 periments upon the matters which are absorbed, but not one good che- 

 mical examination of the nature of the moisture ordinarily found in the 

 soil, and which is the true food of plants. The consequence is, that we 

 know nothing certainly of the processes that go on in the interior of the 

 plant, in nutrition and assimilation. The best thing that has been said 

 on this subject appears to me to be a remark of Liebig, who says that 

 the carbonates of the alkalies are apparently gradually converted into 

 salts of vegetable acids, containing little oxygen. The malates are, 

 through a deoxidising process in the potash and dextrin, probably de- 

 stroyed ; but there is no experimental proof of this. 



Upon the origin of particular compounds we know nothing. Liebig, 

 when he speaks of the possibility and probability of the decomposition of 

 carbonic acid, says that this process must take place, in every case, during 

 the formation of fatty matter. We might admit this if we could form 

 fats out of carbonic acid and water ; but we cannot do this, and all ana- 

 logy leads us to the much more probable conclusion that the fats are 

 formed out of the compounds of the dextrin series. To calculate the vari- 

 ous possible combinations of the elements on paper is not very difficult, 

 but for affording a knowledge of what really takes place in nature, such 

 a proceeding is entirely useless. That some few inorganic compounds 

 are converted into organic compounds during the nutrition of plants, 

 we know with absolute certainty ; that during these changes the inor- 

 ganic salts play an important part, is probable. But what organic com- 

 pounds are first formed, through what special chemical processes they 

 originate, is at present entirely unknown, although it must form the 

 foundation of a true theory of nutrition. In recent times we have re- 

 ceived from the researches of Liebig, Mulder, Dumas, and others, numer- 

 ous schemes and explanations of the various metamorphoses of the 

 organic matters in plants. But by far the most extensive part of the 

 question, and for vegetation generally the most important, has been 

 hitherto untouched : here is a wide field for united exertion. The 

 chemist, often with great industry, builds up a theory which one 

 glance through the microscope dissipates. The physiologist exercises 

 great acuteness in bringing his observations into relation, and when he 

 has done, the chemist tells him it is chemically impossible. Thus both 

 time and energy are lost. 



There is another circumstance to be referred to in this place, which 

 renders observations on plants difficult, and which ought to be regarded 

 in the selection of plants for experiment. Although plants, as such, 

 must exist according to the morphological relation of their physiological 

 elementary organs, yet individuals of one and the same species contain, 

 both qualitatively and quantitatively, a great variety of elements, and 



