PASSAGE OP MATTERS INTO THE ROOTS. r.i) 



without distinction. Upon this assumption, only that 

 substance in the plant could of course pass into it from 

 without, which is withdrawn from the solution within 

 for a formative purpose. 



The investigations made by Schultz-Fleeth show that 

 Nymphcea alba and Arundo phragmites absorb from the 

 same soil and water, the former nearly 13 per cent., the 

 latter 4-7 per cent., of ash constituents ; and of these siUcic 

 acid in tlie most unequal proportion ; the ash of Nym- 

 pha'a alba containing less than ^ per cent, of that substance, 

 while in the ash of Arundo phragmites there are abovQ 

 71 per cent. Upon the supposition just made, an equal 

 amount of sihcic acid is offered to the roots of both plants, 

 and they both take up an equal quantity of it in proportion 

 to the volume of the sap respectively. In the reed plant 

 the silicic acid is incessantly withdrawn fi^om the sap, and 

 deposited in a sohd state in the leaves, the margins of the 

 leaves, the sheaths, &c. As the sap within contains less 

 silicic acid than the solution without, fresh quantities of it 

 are absorbed from the latter ; but not so with the Nym- 

 2?hcea, because the sihcic acid taken up by that plant is 

 not consumed in it. 



If we accept the same reasons for the passage into the 

 plant of carbonic acid and phosphoric acid, then it 

 can possess no actual power of selection, but the permea- 

 tion of the nutritive substances w^ill depend upon osmotic 

 conditions. 



It certainly cannot be denied that the absorption of 

 nutritive substances depends upon growth or increase in 

 mass ; for as it is certain that a plant will not grow if no 

 food is offered to it, so it is equally certain tliat it will 

 absorb no nutriment if the external conditions are not 

 favourable to growtli. Yet tlie view given above would 

 force us to conclusions which are not founded in natiue ; 



