Philosophy of Manures. 339 



be found in excess ; and how is this excess and superfluous 

 matter to be got rid of? Water, and even acids, may he 

 evaporated ; but how will Hme and other alkahes and earths be 

 expelled, unless by excretion ? If silicate of potash, for in- 

 stance, is in greater quantity in the water absorbed by plants 

 than they require, it must be again returned. If we admit any 

 of these are excreted, we must allow a general excretory power 

 of all soluble substances when in excess ; otherwise plethora 

 would be produced, if the excess were deposited and not extri- 

 cated. 



On the views entertained by Professor Liebig, of the Carbon 

 of Plants being exclusively got from the atmosphere by the 

 leaves, and only the fixed ingredients by the roots, Professor 

 Daubeny, at p. 65., seems to lean to the contrary opinion. 

 Professor DeCandolle, in his Vegetable Physiology, lately trans- 

 lated and published in the Gardeiier's Gazette, says that leaves, 

 and all the epidermis of plants, will, under certain circumstances, 

 absorb nutriment, as will animals sometimes by the skin (life 

 has sometimes been preserved for a time by immersion in 

 soups) ; but he altogether repudiates the idea of plants drawing 

 the principal part of their food from the air. Dr. Carpenter, in 

 his excellent work lately published on Comparative Physiology, 

 says that the lower orders of plants, as ^'Igae, &c., are like 

 some of the lower orders of animals, altogether composed of an 

 absorbent substance on the whole surface ; but, in the higher 

 orders of plants and animals, he considers the absorbent surface 

 as confined to the newly formed spongioles of the roots, and the 

 lacteal absorbent vessels of the animal. The principal part of the 

 carbon of plants, however, he considers, is derived from the atmo- 

 sphere, by the leaves inhaling it by the green parts or by the sto- 

 niata, as in respiration. Carbonic oxide may be thus inhaled, as 

 well as carbonic acid. The opinions of theorists appear very unde- 

 cided on this subject. As I stated in my former essay, Dr. Liebig 

 considers the young plant to get its carbon principally by the roots, 

 but to lose this power as it advances in age; though without 

 stating how the constantly renewed young spongioles, always in 

 the growing season presenting a newly formed absorbing surface, 

 possessing all the vital energy inherent in newly formed tissue, 

 can ever lose the property of conveying all the constituents of 

 the food of j)lants. On this subject I enlarged so much 

 before, that I have not opportunity to say much now. I still 

 hold by the opinion, that practice cannot be so far wrong in 

 burying such quantities of carbon in the earth. Manure spread 

 on the ground, though in great measure preserved by the carbon 

 being washed into the soil by rain, has not produced the same 

 effect as that buried to a moderate depth, not out of the action 

 of the heat and air. From the great quantity of carbonic acid 



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