44 CELL-CONTENTS AND CELL-WALLS. 



49. Microchemical Tests for Proteins. All the 



tests that give a colour reaction may be readily used as 

 microchemical methods of detecting proteins in plant 

 tissues. 



(a) Cut thin sections from a Pea, Bean, or Lupin, 

 cotyledon. (1) Treat a section with iodine ; the starch- 

 grains turn blue, the small protein grains turn brown or 

 yellow. (2) Lay a section in strong copper sulphate 

 solution for a minute, rinse in water, and transfer to a 

 little potash in a test-tube, heat to boiling ; mount, cover, 

 and note the violet colour of the protein cell-contents. 

 (3) Apply the xanthoproteic test, by placing a section 

 first in strong nitric acid in a watch-glass, then in strong 

 ammonia ; note the intense yellow colour of the protein 

 contents. (4) Place a section in a little Millon's reagent ; 

 if the protein contents do not turn red quickly, warm the 

 slide. See also 74. 



(6) Cut transverse and longitudinal sections of the grain 

 ( " seed " ) of Wheat and of Maize and apply the above 

 tests. Note that the greater portion of the endosperm 

 consists of cells packed with starch-grains, but the outer- 

 most layer (" aleurone " layer) consists of cubical cells 

 containing protein grains. See also 75. 



50. Protein Grains ("Aleurone" Grains) are found 

 in various parts where food is stored, but are especially 

 abundant and large in seeds. In some cases the grains 

 are small and of simple structure ( 51). In other cases, 

 especially in oily seeds, they are large ( 52) and contain 

 one or more angular " crystalloids " (protein crystals) and 

 also rounded " globoids " consisting of mineral substance 

 (double phosphate of calcium and magnesium). Protein 

 crystals may also occur in cells quite apart from definite 

 protein grains. 



51. Simple Protein Grains. Get dry seeds of Al- 

 mond, Apple, Bean, Pea, Lupiu, Sunflower. Moisten the 

 razor with glycerine, cut sections of the cotyledons, and 

 mount in glycerine. 



