44 



whole root system is bitten or corroded upon the marble. Ex- 

 perimental evidence has shown that the roots, provided they 

 are intact, only absorb gases or liquids : if the root is broken it 

 may absorb small solid matters, but while it remains intact 

 nothing solid enters the plant by its means. 



The salts which are taken up by the plant belong to two 

 groups : 



I. Essential salts, consisting of elements forming an essential 

 part of the organized structure, and used directly to build up 

 the protoplasm and cellulose of the plant; these are formed of 

 the elements Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur. 



II. Non-essential salts, containing elements not in any way 

 essential or capable of replacing essential constituents. They 

 vary according to situation, e.g., Iodine and Bromine, which 

 are found in considerable quantities in sea plants. 



Between these two are a group of elements which go to form 

 part of the plant structure also, and they are important to the 

 plant, as promoting the chemical processes by which the con- 

 version of the group of essential salts from inorganic compounds 

 to very complicated organic ones takes place; as instances of these 

 we have Potassium, Calcium, Magnesium, Iron, and Phospho- 

 rus. They are found in the ash when the plant is burnt. 



It has been found that the ash of plants which grow close 

 together in the same soil, or in the same water, may have a 

 composition which is different in different cases, and which is 

 different also from that possessed by the soil. Hence some have 

 attributed to plants a certain selective or exclusive power, which 

 they exercised by their roots, i.e., that they could absorb certain 

 matters and reject others. This power, if it exists, must be a 

 very feeble one at most, for roots placed in solutions injurious 

 to the plant absorb them ; and the phenomenon is capable of a 

 much simpler explanation by the ordinary laws of diffusion. 

 A salt which is held in solution by the medium, whether soil 

 or water, surrounding the plant, will continue to diffuse into 

 the cells of the roots, until a condition of equilibrium is set up 

 between the two fluids which are separated by the membrane; 

 and this statement is true of those substances which, being in 



