396 Comparatirc Plii/siologij. 



will not account for the whole force of the circulation, though 

 commencing it and being a great assistant ; and the force both of 

 the descending and ascending current, they ascribe greatly to 

 the vital contractile power of the vessels through and among 

 which the sap moves. This opinion is likely to be correct, and 

 it likewise tends to produce more similarity in character 

 between the functions of circulation in plants and animals. 



Liebig and others seem to be of opinion that the particles 

 which have discharged their duty in the structure, and are 

 taken up as waste by the oxygen inhaled, cannot be again 

 rendered subservient to the process of nutrition in the same 

 system ; and if this opinion is correct, if all the waste carbon 

 must be secreted, this would argue the necessity for interstitial 

 absorption in plants as well as animals. DeCandolle and others 

 seem to be of opinion that the carbonic acid formed in this pro- 

 cess may be again decomposed in the leaf, and the nascent 

 carbon be again employed in nutrition. On such a subject 

 there would seem little probability of being able to make de- 

 cisive experiments ; it is more natural, however, to suppose that 

 the effete or worn out particles should be rejected and excreted. 



It is a very general opinion, that chemical changes are begun to 

 be produced in the food so soon as it enters the system of the plant; 

 and this has been said to be corroborated by the augmented pro- 

 portion of the organisable products found in the sap as it ascends 

 the tree. It is uncertain, however, how much of these is derived 

 from the dejoosits of starch, sugar, &c., stored up in the alburnum, 

 &:c., the previous season, to serve for nourishment to the young 

 shoots at the commencement of the growing season ; and un- 

 doubtedly the principal action takes place in the leaves ; it is 

 there principally that the food is prepared for assimilation. 

 From the ramifications of the vascular system, however, sup- 

 plying the air found in them, it is probable that aeration and 

 intei'change of gases may take place, to a certain extent, through 

 the whole of the system of the plant ; an organic action may 

 also be exerted by the membranes of the cells which the fluids 

 pass through. Some are also of opinion that the sap possesses 

 vital proiDcrties, and may itself exert an action on the sub- 

 stances imbibed from the soil ; this may be either in the manner 

 denominated catalytic by Berzelius, in which substances pro- 

 duce chemical changes without being themselves changed, or it 

 may be vital power inherent in the sap or blood. The nitroge- 

 nous excretions from the spongioles probably assist in dissolving 

 the food in the soil, acting like diastase. 



(^To be continued.) 



Erratum. — In the May Number, p. 192., line 15., for "animal" read 

 " mineral." 



