PHYSIOLOGY 43 



and sodium are very important must be worked into 

 the complex molecules which form the basis of life. 

 These mineral salts the plant gets in weak solution 

 from the water in the soil. Curiously enough, although 

 all the solid carbon it requires it can obtain direct from 

 the atmosphere, in which there is such a small percentage 

 of gas containing it, the nitrogen necessary can only 

 be utilised when it is in compounds in solution in water ; 

 and all the abundance of gaseous nitrogen in the air 

 is useless to an ordinary plant. Hence of the manures 

 that must be added to soil that is exhausted by the 

 growth of many generations of plants upon it, those 

 containing nitrogen are of great importance. A few 

 plants are able, with the help of certain bacteria, to 

 obtain nitrogen from the air, and these are to the farmer 

 of the greatest assistance. Clover, Peas, Lupins, and 

 indeed the whole family of Leguminacese, as well as a 

 few trees, have on their roots small swellings which are 

 produced in connection with, and inhabited by, bacteria. 

 There are also in the soil other bacteria which do part 

 of the business of turning the free nitrogen in the atmos- 

 phere which permeates the soil into chemical compounds, 

 which are then further worked up by other bacteria 

 till the clover and other plants with bacterial nodules 

 are able to utilise the resulting mineral solutions. 

 Simple experiments can be made to illustrate the need 

 of plants for the solutions of nitrates, iron, &c,, by 

 growing series of seedlings in glass jars, some in dis- 

 tilled water, which is devoid of any minerals, others 

 in distilled water with all the necessary salts in solution, 

 and others in solutions with one or more of the im- 

 portant salts missing. For instruction in this and the 

 other experiments that can be made to prove the 

 general facts of nutrition and assimilation stated above, 



