ASSIMILATION OF FOOD. 505 



decomposition of the starch or similar matters into wax. It 

 would appear that the exhalation of oxygen, and the absorption 

 of carbonic acid gas, never stand in immediate relation with each 

 other. 



The formation of carbonic acid by other than the green parts of 

 a plant, as the bark and the root, is no vital process, but is the 

 commencement of a process of decomposition (decay). The form- 

 ation of carbonic acid during germination and flowering depends, 

 as in fermentation, upon a decomposition of organic substances ; it 

 thus serves the vital processes without being itself a process of 

 organic development.* The absorption of oxygen, or the oxyda- 

 tion of secreted matters, as the volatile oils, tannin, &c., is entirely 

 independent of the essential life of the plant. 



After a long night of physical and chemical ignorance, the dawn of a 

 true theory of the nutrition of plants is breaking upon us, not without 

 having been dreamed of in the previous darkness. " Plants absorb the 

 crude sap from the soil ; it is then conveyed upwards through the spiral 

 and porous vessels to the leaves, where it is assimilated ; from whence it 

 is sent down to the bark, in order to form buds, leaves, and roots." " The 

 leaves absorb carbonic acid gas, decompose it, and give out the oxygen 

 gas which it contained." 



This was the substance of the dream, of which the least possible pro- 

 mises to be realised ; it was only a dream-picture, not founded on obser- 

 vation or inductive enquiry, and therefore of no value. 



In the first place, there is no such thing as crude sap. It cannot, 

 therefore, be carried to the leaves to become assimilated. From whatever 

 part and at whatever time we examine the sap of a plant, we find that it 

 contains organic principles which cannot come from the soil, because they 

 do not exist there ; such are sugar, gum, malic, citric and tartaric acids, 

 albumen, &c. These substances are diluted with a good deal of water, 

 and mixed with a little carbonic acid and carbonate of ammonia, which 

 are contained in the water of the soil. Even in the cells of the roots, 

 which first receive the moisture of the soil, it is chemically changed, 

 assimilated, and the sap is most decidedly not flowing in special vessels, 

 but passing upwards from cell to cell, and thus it is in every new cell 

 which is being developed by the formative chemical processes ; nothing 

 remains for the leaves to assimilate. That the leaves in their natural 

 growth absorb carbonic acid from the air was a pure invention, for, until 

 Boussingault, no one obtained proof of this by experiment. The fact 

 appears to be fixed by Boussingault, but this proves nothing for the 

 assimilating power of the leaves. From whence then comes this car- 

 bonic acid? Not from the cells in which chemico-vital processes alone 

 are carried on, but from the intercellular passages, which in the largest 

 plants communicate one with another, from the extreme points of the 

 roots. The conclusion that the carbonic acid found in the leaves is con- 

 sumed by them is about as rational as the inference would be, from the 

 respiratory movements of the nose and mouth, that the brain performed 



* At the meeting of the British Association at Cambridge in 1845, I read a paper 

 before the Section of Natural History, the principal object of which was to give the 

 explanation alluded to in the text, of the facts which occur in the germination of plants. 

 See Transactions of Brit. Ass. for Adv. of Sc., 1845. TRANS. 



