regulatmg the Vital and Physical Phenomena. 335 



that, for its commencement and continuance, a constant differ- 

 ence is necessary in the density of the fluids on the two sides 

 of the septum. This difference appears to be produced in the 

 living plant by the admixture of elaborated or descending, with 

 the crude or ascending sap ; and in consequence, a portion of 

 the former is excreted from the roots by the action of exosmose. 

 The cessation of this action of admixture (a change obviously 

 dependent upon vital actions) at the death of the plant, fully 

 accounts for the non-continuance of endosmose, which might 

 also be occasioned by the facility with which the delicate tissue 

 of the spongioles undergoes disorganizing changes. It is also 

 evident that the continuance of the vital processes going on in 

 the leaves is necessary for the removal of the imbibed fluid as 

 fast as it is absorbed by the spongioles, just as the combustion 

 of the oil at the apex of the wick is necessary for its continued 

 rise by capillary action beneath ; * and it is now acknowledged 

 that the motion of the sap in spring commences at the extre- 

 mities of the branches, owing to the vital actions which result 

 from the stimulus of heat and light, the endosmose not occur- 

 ring at the roots until the superincumbent column of fluid has 

 been drawn off. If these views be correct (and they are not 

 negatived by the few instances of selective absorption which 

 occur in plants, as these are readily explicable on this theory 

 (Edi?i. Med. and Surg. Journ. xlviii. p. 24), it follows that we 

 are to regard absorption as a change resulting primarily from 

 the physical properties of the tissue, but dependent for its con- 

 tinuance upon actions of a vital nature in other parts of the 

 organism. 



25. In the early stage of vegetable existence, before the evo- 

 lution of the organs, whose object is to convert the crude ali- 

 ment into organizable materials, the embryo is supplied with 

 nourishment already prepared by the parent. When, how- 

 ever, the plant has arrived at its full growth, its aliment is de- 

 rived from the simplest materials, viz. water, carbon, and, in a 

 few instances, azote ; and it is in the conversion of these into 

 the organic compounds which are found in the nutritious juices 

 of plants, that we may recognise the action of physical laws, 



• Henslow's Botany, p. 179. 

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