174 CHEMICAL COMPOSITION OF PLANTS 



the particular abundance of the element in growing tissues is 

 thus partially explained. 



It has been observed that the ' fixation of carbon ' in green 

 tissues ceases when potassium is absent, and cereals and peas grown 

 with an insufficient supply produce small thin grains and seeds. 



The place of potassium in the economy of the plant cannot be 

 taken by any of the other nearly allied elements such as sodium 

 and lithium. 



(vii) Calcium. — Fungi appear to be able to dispense with calcium, 

 but for green plants it is an essential element. It is absorbed 

 from the soil in the form of a nitrate, phosphate or sulphate. 



In the young parts of plants calcium is generally present in small 

 quantity only and in some instances it may be missing altogether 

 from such parts for a time, its absence leading to no apparent in- 

 jurious effect. It is most abundant in the older parts of plants, such 

 as fully-developed and dying leaves, bark and pith, and occurs in 

 the form of salts of organic and inorganic acids more especially as 

 oxalate and carbonate. The amount of lime (CaO) in the ash of 

 barley, oat, and wheat straw is generally about 7 per cent. 



Although seedUng plants may continue to grow for one or 

 two months without calcium, they always appear stunted under 

 such conditions and present other features of ill-health ; if calcium 

 compounds are still withheld death takes place. 



Like some other essential elements calcium plays a many- 

 sided role in plant-nutrition. 



Oxalic acid and soluble oxalates are formed in certain plants 

 and when present in very sHght excess act injuriously upon the 

 nucleus and other cell-constituents ; in the presence of calcium 

 salts their accumulation and poisonous action is prevented by the 

 formation of insoluble calcium oxalate. 



Calcium is, however, not exclusively utilised for the neutralisa- 

 tion of oxalic acid, for there are many plants which never contain 

 oxalic acid, and yet it is found that such plants still require this 

 element for perfect growth. The assumption that calcium oxalate 



