VEGETABLE CYTOLOGY 
89 
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, nucleins , 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 1 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.” 
