VEGETABLE CYTOLOGY 



8 9 



19. Proteins. Proteins are complex nitrogenous substances 

 forming the most important of the reserve foods of plants. They are 

 found in all the living and many of the dead cells of plants, although 

 most abundant in seeds. Protoplasm, itself, is composed largely 

 of these substances. They all contain carbon, hydrogen, oxygen, 

 nitrogen and sulphur, and many contain in addition phosphorus. 

 They are formed by the addition of nitrogen, sulphur and fre- 

 quently phosphorus to elements of grape sugar. The nitrogen, 

 sulphur and phosphorous elements are obtained from nitrates, 

 sulphates and phosphates which are dissolved in the water taken 

 in through the roots. The names of proteins recorded may be 

 found by the hundreds. These are grouped into chemical classes, 

 the most important of which from the standpoint of their occurrence 

 in plants are the globulins, albumens, glutelins, nudeins, and gliadins. 

 Of these the globulins are found most extensively. Globulins are 

 insoluble in water but soluble in sodium chloride solutions. They 

 do not coagulate upon the application of heat. 



Albumens are soluble in water and coagulate with heat. 



Glutelins are insoluble in water, sodium chloride solution and 

 strong alcohol. 



Gliadins are nearly or wholly insoluble in water but soluble in 

 70 to 90 per cent, alcohol. 



Nucleins are insoluble in water but soluble in akaline solutions. 



The following tests are of value in determining the presence of 

 proteins. 



Lugol's solution stains proteins yellow or brown. 



Concentrated nitric acid stains proteins yellow. This color 

 becomes deeper upon the addition of ammonia water. 



Million's reagent stains proteins a brick-red. 



Concentrated solution of nickel sulphate colors proteins yellow 

 or blue. 



If sections are placed for an hour or two in a solution of i Gm. of 

 sodium phospho-molybdate in 90 Gm. of distilled water and 5 

 Gm. of nitric acid, the proteid substances appear as yellowish 

 granules. 



The globulins (phytoglobulins) frequently occur in bodies called 

 "aleurone grains." 



