52 THE WORK OF THE ROOTS [chap. 



salts taken from the soil. Phosphorus, again, must 

 enter the plant as a phosphate or phosphoric acid ; 

 sulphur as a sulphate, for sulphites, sulphides, and many 

 other compounds of sulphur are not only useless but 

 poisonous. The bases found in the plant enter as the 

 neutral salts of sodium, potassium, calcium, magnesium, 

 etc. ; in fact, for all the elements except chlorine we may 

 say that the plant prefers or even requires the ordinary 

 most highly oxidised compound of each element. It 

 will thus be seen that the plant behaves towards its 

 nitrogen and its ash constituents just as it does towards its 

 carbon compounds — i.e.^ it begins with a simple oxidised 

 compound, reduces it by splitting off oxygen, and builds 

 it up into a variety of bodies of great complexity 

 possessing more potential energy than the initial com- 

 pounds out of which they have been wrought. It is 

 easy to understand that a plant must take up the 

 mineral substances which are found in its ash from the 

 soil, because it is in contact with no other source of such 

 a constituent as phosphoric acid. The case is, however, 

 different as regards nitrogen, because the plant when 

 growing is surrounded by the atmosphere, four-fifths of 

 which consists of nitrogen in a free, uncombined state. It 

 has therefore been a question of much interest — one that 

 has always been in dispute, and is still not regarded as 

 settled by some people — whether the plant is not able to 

 utilise some of this vast stock of free nitrogen and draw 

 a part, if not the whole, of the combined nitrogen it 

 contains from such a source. The evidence is, however, 

 strongly against the view that plants are in any way 

 able to " fix " or bring into combination the nitrogen gas 

 of the air, except in one or two special cases where the 

 process is really effected by certain bacteria living in 

 partnership with the plants ; the general run of plants 

 are supposed to be wholly dependent on combined 



