170 BIOLOGY OF PLANTS. 



which require soda, iodine, or some substance yielded by the 

 sea, as common salt. Such plants will follow the salt water, 

 wherever it is found. If salt-works are opened in the inte- 

 rior of a country, the sea-weeds will find the spot, and mi- 

 grate to it. 



The absolute necessity of inorganic bases to the perfect 

 development of plants, is shown by the fact, that each species 

 of plant produces organic acids, as the acetic, tartaric, ma- 

 lic, etc. These acids are united with bases, either organic or 

 inorganic, and the latter is the case, in most instances. The 

 quantity of these acids can be accurately ascertained in each 

 species of plant; and as their power of saturation is known, 

 the quantity of inorganic bases may be accurately deduced. 

 It must always bear an exact ratio to the organic acids. The 

 quantity of these acids varies according to the nature of the 

 soil, in order to suit the different organic bases. 



As the roots of plants, like a sponge, imbibe from the 

 soil whatever substances* are held in solution by water, 

 it is evident, that many of the inorganic bases and other mat- 

 ters will be introduced into the organs of the plants, which 

 cannot be assimilated. These substances are again returned 

 to the soil. This process has been inferred from observation. 

 From the nature of the case, we have a strong presumption 

 in favor of its truth. It may, at least, explain very many 

 phenomena of vegetation. Macaire Princep has shown by 

 experiment, that plants made to vegetate with their roots in 

 a weak solution of acetate of lead (sugar of lead), and then 

 in rain-water, yield back all the lead absorbed. So, also, 

 when a plant is sprinkled with nitrate of strontia, it will ab- 

 sorb it by the leaves, but return it all to. the soil ; hence, we 

 may color a plant with various substances;! but, after a 



* The roots do appear to possess some power of discriRiination, as 



they will imbibe different quantities of substances which are presented. 



t " When the soil, in which a white liyacinth is growing in a state 



