TEMPORARY MOUNTS AND MICROCHEMICAL TESTS 83 



considering the various stains we shall indicate the effect of each stain 

 upon the various plant structures. 



Starch. — Mount the starch or starch-containing structures in water, 

 and allow a drop of iodine solution to run under the cover. Starch 

 assumes a characteristic blue color. The solution may be prepared by 

 dissolving 1 g. of potassium iodide in 100 c.c. of water and adding 0.3 

 g. of sublimed iodine. A strong solution of iodine in alcohol (about 1 g. 

 in 50 c.c. of absolute alcohol) keeps well. A drop of this solution added 

 to 1 c.c. of water is good for testing. With too strong a solution, the 

 starch first turns blue but rapidly becomes black. 



Grape-sugar. — In cells containing grape-sugar, bright-red granules 

 of cuprous oxide are precipitated by Fehling's solution. It is better to 

 keep the three ingredients in separate bottles, because the solution 

 does not keep long after the ingredients are mixed. The solutions may 

 be labeled A, B, and C. 



j Cupric sulphate 3 g. 



1 Water 100 c.c. 



j Sodium potassium tartrate (Rochelle salt) ... 16 g. 



\ Water 100 c.c. 



p / Caustic soda 12 g. 



\ Water 100 c.c. 



When needed for use, add to 10 c.c. of water 5 c.c. from each of the 

 three solutions. The sections, which should be two or three cells in 

 thickness, are warmed in the solution until little bubbles are formed. 

 Too much heat must be avoided. Mount and examine in a few drops 

 of the solution. The twig or organ may be treated with the solution, 

 and the sections may be cut afterward. Other substances pi-ecipitate 

 copper, and may be mistaken for grape-sugar by the beginner. 



Cane-sugar. — Cuprous oxide is not precipitated from Fehling's so- 

 lution by cane-sugar, but after continued boiling in this solution the 

 cane-sugar is changed to invert-sugar and the copper is precipitated. 

 The solution becomes blue. 



Proteins. — The proteins turn yellow or brown with the iodine solu- 

 tion. It is better to use a stronger solution than when testing for 

 starch. It must be remembered that many other substances also turn 

 brown when treated with iodine. 



When proteins are warmed gently in concentrated nitric acid, the 

 acid becomes yellow. The color may be deepened by the addition of a 

 little ammonia or caustic potash. 



