An Introduction to Zoology. 29 



very first herb called into existence, and so constituted as to yield seed 

 after liis kind, thus propagating the species, while the individual perishes, 

 and so renewing from time to time the occupants of the face of the earth. 



The process of nutrition requires, first, the introduction of a specific 

 nutritive fluid ; secondly, the bringing that fluid into contact with the 

 vivifying influence of the air j thirdly, its dissemination through the several 

 parts of the system ; and fourthly, its assimilation and conversion into 

 various forms, each differing very materially both from its original condi- 

 tion and from the others, but accommodated according to its relations to 

 the parts which it is destined to renovate. 



These four processes, or stages, are in vegetables exceedingly simple, but 

 in animals much more highly complicated ; because in vegetables the nutri- 

 tive fluid or sap, being in its original state only water, holding in solution a 

 number of earthy, and saline, and gaseous particles, is readily furnished 

 by the soil from which they grow, and needs not, as in animals, be con- 

 cocted by a long course of previous preparation. The water thus laden is 

 absorbed by the spongioles, or spongiform extremities of the roots, and 

 rises through the stem, (or rather, through the void spaces, between the 

 minute cells of which the porous substance of that stem consists,) partly 

 by the mere mechanical action of capillary attraction, and partly, it is 

 probable, through the vital coutractibility of the mass. 



The aeration of the sap is conducted in the leaves, which may therefore 

 be considered as vegetable lungs ; for this process of aeration is remarkable 

 both for its analogy and for its opposition to animal respiration, by means 

 of which, by a most admirable adjustment of nature, the antagonist (if we 

 may so speak) modifications of a similar function, are constantly made to 

 balance one another in their effects upon atmospheric air, upon which both 

 act, and thus tend to preserve it in that state of uniformity which is among 

 the most necessary conditions of nature. For in the animal lungs, the 

 oxygen inspired uniting with the carbon, with which the venous fluid 

 becomes surcharged in its passage through the system, abstracts it in the 

 form of carbonic acid gas. In this state the air soon becomes unfitted for 

 further inspiration : but that which thus converts it into a poison for 

 animal life, accommodates it as a rich source of nourishment for vegetable 

 life, for carbon forms the principal sustenance of plants : during the 

 presence of the light of day, the leaves have the power of decomposing the 

 carbonic acid wafted to them by the atmosphere, thus tainted by animal 

 respiration, and restoring to it the pure oxygen ; while the carbon, being 

 placed apparently in a state of atomic division, which fits it for uniting 

 with vegetable compounds, is absorbed into the vegetable circulation, at 

 the moment when it is in a state of nascent disengagement from its former 

 UDion with oxygen; and although, when the solar light is withdrawn, the 

 leaves, losing this power of decomposition, can only absorb the oxygen and 

 unite it with the carbon which they contain, yet their former energy is 



