180 Plant Physiology 



contrasted, the last-named in each case enduring much 

 less acidity. 



Lime is important in effecting a liberation of (by 

 rendering available) other nutrients, and on this account 

 it should be used cautiously, in order that waste by leach- 

 ing may not result. 



It is important in maintaining phosphates in available 

 form, and in counteracting the injurious effects of many 

 substances in the soils, including certain products of fer- 

 tilizers. In many ways it has an intimate relation to the 

 nitrogen supply of plants, for it promotes the formation 

 of nitrates from organic matter, diminishes the destruction 

 of these, and seems to be generally almost indispensable 

 for the proper development of the nitrogen-fixing root- 

 tubercle organisms. 



The above effects may be considered those of most 

 intimate consequence for plants generally ; but in addition 

 it may improve (by nocculation) heavy soils, and it may 

 be important as an insecticide and a fungicide (although 

 it is favorable to potato scab and to root rot of tobacco). 



96. Iron. - - A certain amount of iron seems to be 

 necessary as one of the factors in the normal development 

 of leaf green, or chlorophyll, although it is not regarded 

 as a constituent of the organic bodies which make up this 

 substance. Lack of iron is one of the many conditions 

 leading to pathological chlorosis. It may be that the lack 

 of iron affects the protoplasmic structure (the plastid) 

 in which the chlorophyll is deposited, for the best evidence 

 points to the use of iron by every living cell, including, 

 therefore, those organisms which contain neither this pig- 

 ment nor any allied compounds. 



