Comparative Physiology. 385 



is the principal characteristic by which the action of leaves can be 

 compared to that of animals ; the circulation of the fluid being 

 produced by the power of endosmose and contractility on the 

 ascending sap, and that of contractility, gravity, and endosmose 

 on the descending. Digestion aj^pears no part of their action, 

 the absorption of light aj)pearing only an assistant in the much 

 greater chemical action required by plants than animals. The 

 action of plants consisting in the preparation of ternary com- 

 pounds, as gum, sugar, starch, lignin, &c., and quaternary com- 

 pounds, as fibrine, albumen, casein, &c., from binary compounds, 

 as water, carbonic acid, ammonia, &c., necessarily implies more 

 chemical action than that of animals, whose food is principally 

 in an organised state, already fit for assimilation, or at least 

 identical in comjjosition with most of the animal tissues, unless 

 the nervous, &c. The decomposition of carbonic acid, water, 

 and ammonia into their elements, and their recombination in 

 a state fit for assimilation by the different organs, require a 

 very great degree of chemical power; and hence full expo- 

 sure to the direct heat and light of the sun is necessary to plants, 

 to assist the organic action of the leaves in producing these 

 results. 



The vital force, Liebig says, is not needed so much in plants as 

 in animals, for the [)i"eservation of the tissues from oxygenation ; 

 the non-azotised portions of the tissue may be reckoned as 

 compai-atively destitute of susceptibility to oxidation, when 

 compared with the azotised portions : hence, he says, the vital 

 force of plants is principally expended in the preparation of new 

 matter, and not wasted as in animals by voluntary and involun- 

 tary motions and presei'vation of tissues ; there is therefore 

 more available vital force, and plants are more capable of aug- 

 mentation in bulk and of forming new matter. He likens vital 

 force, in its developement (not in its character, which he says is 

 distinct), to that of galvanism ; as the action of the zinc and 

 acid produces, when in action, a force which may be collected 

 and transmitted along iron rods, so is vital force generated, he 

 says, from oxygenation, and preserved to assist in voluntary and 

 involuntary motion, being transmitted along the nerves to 

 Avhere it is needed. It has not been customary to talk of vital 

 force as a determinate quantity, increasing from the want of 

 motion in one quarter, and being transmitted to another 

 wliere motion is needed ; but the explanation is plausible : the 

 less waste and motion in plants may accumulate vital force, and 

 the almost indefinite capability of extent in plants, as compared 

 with animals, is well known to all practical men, whether it 

 may flow from accumulated vitality or Avhatever cause. Miiller 

 says : " Plants, having only one mode of manifesting life, namely, 

 by vegetation, do not require manifold organs in addition to 



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