ROTATION OF CEOPS. 



133 



ticular salt is not present, the plant frequently takes up an isomor- 

 phous one. 



The differences in the absorption of solutions depend, perhaps, on 

 the relative densities alone, and not on any peculiar selecting power 

 in roots, for it is well known that poisonous matters are absorbed as 

 well as those which are wholesome. The following experiments show 

 that poisonous matters in solution, varying from half a grain to five 

 grains in the ounce of water, are taken up by roots, and that some 

 substances which are poisonous to animals do not appear to act 

 energetically upon plants : — 



Ziuoio cHoride . c 

 Zincio STilphate 

 Cuprio sulphate 

 Cupric nitrate 

 Cuprio acetate 



Mercuric chloride . 



Arsenions acid 

 Potassic aiseniate . 

 Phimbic acetate 



Potassic "bichromate 



Ferrous nitrate and I 

 sulphate J 



Baric chloride 

 Baric nitrate . 



Stroutic nitrate 



Calcic chloride, sul- ) 

 phate, and nitrate \ 



Magnesic chloride and (_ 

 sulphate f 



Sodic phosphate 



Sodic chloride 



Growing Plants. 

 1 beans 

 cabbages and ■whestt 

 beans 



beans 

 wheat 



quickly destroyed. 



cabbages and wheat 

 barley and cabbages 

 beans 



cabbages, beans, barley 4 

 beans 



cabbages and wheat 

 beans 



beans 



beans and cabbages 



beans and cabbages 

 beans and cabbages 



weak solutions did not de- 

 stroy. 



destroyed in a few days. 



destroyed unless much di- 

 luted. . 



destroyed in a few days. 



quickly destroyed. 



plants uninjured, unless so- 

 lution strong. 



improved when very di- 

 luted. 



injui'ed, and if strong de- 

 stroyed. 



no injury when diluted. 



EoTATioN OF Ceops. — As the inorganic materials which enter 

 into the composition of plants vary much in their nature and relative 

 proportions, it is evident that a soil may contain those necessary for 

 the growth of certain species, while it may be deficient in those re- 

 quired by others. It is on this principle that the rotation of crops is 

 founded ; those plants succeeding each other in rotation which 

 require different inorganic compounds for their growth. In ordinary 

 cases, except in the case of very fertile virgin soil, a crop if grown for 

 several years in succession in the same field will deteriorate in a 

 marked degree. This has been tested by growing plants on the same 

 and on different plots in successive years, with the following 

 results : — 



