Comparative Physiology. 465 



is not presently needed being deposited in the state of fat, for 

 future exigencies. Plants, on the contrary, require great che- 

 mical decomposition and recomposition, their food consisting 

 principally of inorganic substances. It is, however, carrying 

 simplification too far, to assert that organised substances form 

 no part of their food. When organised substances are so far 

 broken down by chemical decomposition that they become so- 

 luble, they will undoubtedly be absorbed and converted into 

 food. I lately stated, in a former essay, the opinions of 

 Professor Gazzin, that straw and other vegetable matter, 

 broken down into small j)ieces by putrefaction, formed a main 

 source of the food of plants. DeCandolle, also, says that 

 soluble organic matter is taken up by the water absorbed, con- 

 verted into carbonic acid in the plant, and this again decom- 

 posed in the leaf; carbon not being available, he states, in 

 forming the sap of the plant, unless recently extracted, or in 

 Avhat is called a nascent state. Miiller, page 40., says : " It 

 appears from the experiments of Hassenfratz, Saussure, &c., that 

 j)lants grow very imjDcrfectly in carbonic acid and water alone, 

 and that it is only when they are at the same time nourished by 

 organic compounds in solution, which have not icholly undergone 

 decomposition, that plants generate organic matter from binary 

 compounds." The old opinion, that plants get their carbon 

 ivholly from the air, has lately been greatly upheld by the 

 writings of Liebig, Dumas, and others ; who, perhaps from a 

 wish to simplify the subject, have given the weight of their 

 opinions on that side. The former admits, however, that young 

 plants get their carbon by the roots from the soil ; and the latter 

 notices the immense quantity of carbonic acid found by Bou- 

 cherie to issue from the trunks of some cut down trees as 

 rendering the subject doubtfid. Schlieden, one of the most 

 profound physiologists, seems to be of opinion that it is prin- 

 cipally got by the roots from the soil, and appears doubtful of 

 much being got from the air by the leaves. The water of the 

 soil, he thinks, is generally saturated with carbonic acid; at 

 ordinary pressure the water is capable of containing a quantity 

 equal to its own bulk, and, where the pressure is considerable, 

 much more. Dr. Mohl is said, in the Chronicle of 20th May, 

 to have lately published similar opinions. It has been said that 

 the fact of plants growing on bare rocks, newly thrown up by 

 volcanic agency, is a proof of their getting most of their carbon 

 from the air. The growth of such plants, however, cannot be 

 called vigorous, and, besides, great part of their carbon is pro- 

 bably got from the rain water which feeds them washing down 

 the carbonic acid of the air; volcanoes form also a great source 

 of carbonic acid ; it may abound in the fissures of such rocks, 

 and every little pool of water on the rock will absorb carbonic 



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