164 EXPERIMENTS WITH PLANTS 



Why? We notice that the longer the seed-leaves re- 

 main on the plant the more they shrivel and lose sub- 

 stance. It looks as though the plant were absorbing 

 the substance of the seed-leaf to obtain food for its 

 growth. 



Do the seed-leaves contain food substances? The 

 principal food substances needed for the growth of 

 plants and animals are (a) starches and sugars, (h) 

 fats and oils, and (c) proteids (substances like white 

 of ^^g). Let us test the seed-leaves to see whether 

 they contain these substances. 



Test for starch by means of iodine solution, made 

 by dissolving potassium iodide^ in water (about one 

 part to seventy-five of water), and adding iodine crys- 

 talsMmtilthe solution becomes dark brown in color. 

 Apply the solution to a freshly cut or broken surface 

 of the seed-leaf (if the seed-leaf is dry it will take 

 the solution some time to soak in). A dark blue or 

 blackish coloration indicates the presence of starch. 



Sugar, if present in sufficient quantity, may be 

 detected by taste (be careful not to taste poisonous 

 seeds, e. g.. Castor-beans) ; if none is detected by this 

 means, use Fehling's solution, which can be made up 

 by a druggist. 2 The seed-leaf to be tested is cut up 

 into thin slices and boiled for a moment or two in the 



1 Obtainable at drug stores. 



2 This can be made up by any druggist. Dissolve .34.65 grams of purified 

 copper sulphate in 200 cc. of water to make Solution I. Dissolve 173 grams of 

 sodium potassium tartrate (Rochelle salt) in 480 cc. of 10 per cent sodium 



