THE PLANT 9^ 



importance, except in the case of a few plants — e.gr., buckwheat, 

 mangolds and cabbages. 



Potassium is absorbed as various soluble salts. It generally 

 occurs in the plant in union with organic acids, which decompose 

 on burning, leaving potassium carbonate in the ash. In some 

 plants the sap contains nitrate, chloride and sulphate of potash. 

 Potash compounds appear to be necessary for the production 

 of starch, sugar and other carbohydrates, and are always most 

 abundant in the leaves and young shoots. 



Calcium exists in union with organic acids, and aids in the 

 conversion of starch into sugar. In many cases it appears to 

 act beneficially in converting vegetable acids into insoluble 

 compounds, which are deposited in the plant — e.g.., calcium 

 oxalate. It, too, is found largely in the leaves. 



Magnesium is distributed over all parts of the plant, but 

 little is known of its functions. It has recently been shown to 

 be a constituent of chlorophyll. 



7ron, though indispensable, usually occurs in very small 

 quantities. It is essential for the production of chlorophyll. 



Sodium, though always present in the ash, does not appear 

 to be essential to the plant. It cannot replace potassium. 



The metals above mentioned also act as carriers of nitric acid. 

 When nitrate is absorbed by a plant the nitrogen is used in the 

 formation of albuminoid substances, while the bases unite with 

 organic acids. When the plant is burnt the metals are left as 

 carbonates, and it is found that, the richer a plant is in nitrogen, 

 the larger is the amount of bases left as carbonate in the ash. 



YI. Albuminoids or Proteids. — These are substances which 

 resemble albumin or white of egg. They form a large class of 

 bodies, which differ in physical properties — e.g., solubility and 

 coaf'ulability — are of highly complex composition or consti- 

 tution, and contain carbon, hydrogen, oxygen, nitrogen and 

 sulphur. They occur in all living matter, being essential 

 constituents of protoplasm. Their composition varies some- 

 what, the following being the usual limits : 



