14 PLANT LIFE ON THE FARM. 



ripening of seeds, while its effect on the growth of turnips 

 is familiar to all practical men. 



Iron is essential to the formation of leaf-green "chloro- 

 phyll " and chlorophyll is essential to the production of 

 starch ; hence iron in some shape is essential to plants, 

 and it also is supplied from the soil in the form of saline 

 solutions. 



What precise function lime plays in the plant's economy 

 is not known ; hut indirectly it is of importance as a means 

 of introducing phosphorus and other essential ingredients. 

 At Rothamsted, in two of the plots upon which harley has 

 been grown for thirty years in succession, a mineral manure 

 with nitrogen has heen applied ; hut in the one case lime 

 has heen added, in the other (otherwise treated exactly in 

 the same way) no lime has been added. The plants on the 

 plot without the lime are always of a darker green colour, 

 but they are relatively deficient in carbon. Under equal 

 conditions, it is seen that the amount of carbon assimi- 

 lated from the atmosphere in the manner to be hereafter 

 mentioned is direcLly dependent on the amount of available 

 nitrogen, which latter is derived from the soil (Lawes and 

 Gilbert). 



It must not be forgotten that the substances we have 

 mentioned, as well as others not alluded to, though possibly 

 not directly concerned in the nutrition of the plant, yet 

 are so indirectly by causing changes in the soil, by render- 

 ing some matters soluble and capable of osmotic absorption 

 which would not be so without their aid, by storing up 

 and preventing the waste of ingredients useful as plant 

 food, and so forth ; but these matters pertain rather to the 

 physical and chemical history of the soil, on which account 

 they may be passed over here without further mention. 



