82 NUTRITION. 



osmose often exist, as a philosophical writer before 

 quoted remarks, as actions going on in living structures 

 causing in them motions of their fluids, it is equally 

 true that they exist even in unorganized structures, and 

 as a consequence of which no strictly vital changes are 

 seen to follow ; and also that vital phenomena seem- 

 ingly in close analogy to what we are at present refer- 

 ing to, often exist when the conditions of endosmose 

 and exosmose never do : and further we know the vital 

 phenomena, which it undertakes to explain, are liable to 

 great and often sudden changes and modifications from 

 various stimuli and sedative poisons, and from injuries, 

 and which have no such effect upon this or upon any 

 other purely physical principles. In regard to the view 

 of Du Petit Thouars, one difficulty, it appears to me, 

 may be stated ; if the buds swell in the spring by 

 warmth producing rarefaction, (for they cannot by the 

 entrance of more sap into them, as it is supposed not to 

 have begun to move yet), the fluids contained in and 

 near them will be rarified too, and therefore instead of 

 attracting the circumjacent fluid to fill up vacant space, 

 and so give rise to the ascending motion, it would oc- 

 cupy larger space than it did before, and press the rest 

 down towards the roots. Finally, we know that the 

 motion of the sap takes place most vigorously when 

 transpiration is at its lowest ebb, likewise in plants grow- 

 ing under water ; and also that the absorption and mo- 

 tion of the sap in some cases greatly exceed anything like 

 the transpiration which takes place, as shown by Hum- 

 boldt, in which an Agave when cut yielded 200 cubic 

 inches of fluid in twenty-four hours, whilst in this plant 

 transpiration is very little indeed. 



50. In regard to this subject, in our present state of 

 knowledge, I believe we must allow that the ascent and 

 motion of the sap, though materially modified by many 

 of the circumstances already mentioned, is one of those 

 actions resulting from a combination of certain vital 

 principles, one of which is vital affinity, by which the 

 elements of the nutritious matter must be thrown into 



