METABOLIC PROCESSES IN THE PLANT. 249 



Representatives of the third group of vegetable proteids are 

 present in specially large quantities in wheat flour. This is mixed 

 with water, and the paste well kneaded between the hands under 

 a continuous fine stream of water. There remains behind a tough 

 elastic mass of glutin with which only small quantities of starch 

 are mixed. Glutin, which is soluble in water containing potash, 

 consists of a series of proteids (glutin-proteids), viz. glutin-fibrin, 

 gliadin, and mucedin, which can be partially isolated from it by 

 means of alcohol. 2 



1 For more detailed information see Detmer, in Wollny's Forschungen avf 

 dem (rebiete der Agriculturphysik, Bd. 2, Heft 4. I do not here deal with WejFs 

 researches on vegetable casein (see Detmer, Lehrbuch der Pfianzenpliysiologie, 

 1883, p. 157). 



2 For further details consult Kitthausen, Die Eiweissstoffe der Getreidearten, 

 1872, p. 28. 



94. Macrochemical and Microchemical Reactions of Proteids. 



To familiarise ourselves with one of the more important micro- 

 chemical proteid reactions (the biuret reaction), an aqueous solu- 

 tion of albumin, or water holding in suspension conglutin from 

 lupin seeds, is heated to boiling, a small quantity of caustic soda 

 solution is added, and into the hot fluid is introduced, by means of 

 a glass rod, a drop of Fehling's solution. The presence of pro- 

 teids is indicated by a violet coloration of the fluid. To prepare 

 Fehling's solution 34'65 gr. of Copper sulphate, purified by re- 

 crystallisation, are dissolved in 200 c.c. of water. 'Also 173 gr. 

 of Sodium Potassium tartrate are dissolved in 480 c.c. of a solu- 

 tion of caustic soda of sp. gr. 1'14 (about 10 per cent, soda lye). 

 These two solutions are now mixed and diluted up to 1000 c.c. at a 

 temperature of 15 C. 



In the cells of the parenchyma of Phaseolus cotyledons there 

 are present, besides starch-grains, large quantities of proteids. We 

 may therefore conveniently employ sections (which must be at 

 least two cells in thickness) of bean cotyledons in order to 

 familiarise ourselves with the microchemical reactions of proteids. 

 We pour into an evaporating dish a few cubic centimetres of con- 

 centrated Copper sulphate solution, 1 or a solution of Cuprio 

 tartrate, 2 prepared by mixing a solution of 5 parts of Copper 

 sulphate with a solution of 9 parts of normal Potassium tar- 

 trate, and filtering off the relatively somewhat insoluble Potas- 



