64 THE PLANT. 



These plants received their incombustible nutritive 

 substances from a solution in which these elements are 

 most uniformly mixed and diffused ; and yet a com- 

 parative analysis of the water and the ash-constituents 

 of these plants shows that each species absorbs from the 

 same solution different quantities of potash, lime, silicic 

 acid, and phosphoric acid. 



The ash of duckweed was found to contain 22 parts 

 of potash to 10 parts of chloride of sodium, whereas the 

 water in which the plant had grown contained only 4 

 parts of potash to 10 parts of chloride of sodium. In 

 the plant the relative proportion of the sulphuric acid 

 to the phosphoric acid was 10 to 14 ; in the water, 10 

 to 3. 



It is quite the same with marine plants. Sea-water 

 contains for- 25 or 26 parts of chloride of sodium 1'21 to 

 1*35 of chloride of potassium ; but the plants growing 

 in it contain more potash than soda. The kelp of the 

 Orkney Islands, which consists of the ashes of many 

 species of fucus,* contains for 26 per cent, of chloride 

 of potassium only 19 per cent, of chloride of sodium. 



Sea-water contains manganese, but in such exceed- 

 ingly small quantity that it would certainly have 

 escaped analysis, were it not invariably found among 

 the ash-constituents of many marine plants. The ash 

 of jPadina pavonia (a species of tang) is found to con- 

 tain of this mineral even more than 8 per cent, of the 

 weight of the dried plant.f 



By the same power of selection the laminaria with- 

 draw from the sea-water in which they grow the iodine 

 compounds present in it in such exceedingly minute 

 quantities. Chloride of potassium and chloride of sodi- 

 um have the same form of crystallisation, and so many 



* See Godechen's analysis of the ash of different species of fucus. 

 (< Annal. d. Chem. und Pharm.' liv. 351.) 



f To give some idea of the extraordinary power which this plant must 

 possess to withdraw the manganese from sea-water, I need simply state 

 that the quantity of this metal in sea-water is so exceedingly small, that I 

 could find distinct traces of it only by subjecting the sesquioxide of iron, 

 obtained from twenty pounds of sea-water, to a most searching analysis. 

 (Forchhammer and Poggendorff, xcv. p. 84.) 



