THE FOOD OF THE PLANT 51 



formed, but, on the addition of more ammonia, the 

 precipitate is dissolved and a blue solution is formed. 



The red solution is prepared by dissolving potassium 

 bichromate in water. A saturated solution should be 

 made. This substance is extremely poisonous. 



The double-jacketed bell-jars are very convenient for 

 this experiment, but they are not essential and they 

 are expensive. 



To carry out the experiment without the double bell- 

 jar it is well to use a water plant. Elodea canadensis, 

 the American pond-weed, serves the purpose Avell. 



Some pieces of Elodea are depleted of starch by being 

 kept in the dark in the usual way. They are then 

 placed in two bottles of water in which 

 IS small shot acting as ballast. The 

 bottles are corked and the joints thor- 

 oughly waxed. The coloured solutions 

 are put into two large glass jam or 

 pickle jars, and the small bottles con- 

 taining the Elodea lowered into the solu- 

 tions (Fig. 25). As before, the whole 

 apparatus must be put into a good light. 



Observations. — When the usual starch 

 test is applied it will be found that in j-jq 25 



each case a great deal of starch has 

 been formed by the plant in the red light, but very 

 little by the plant which has been subjected to the 

 blue rays. 



Inference. — The red rays of light are more effective 

 than the blue in starch formation. 



Experiment 32 



Aim. — To find out whether the formation of starch 

 is influenced by temperature. 



Method. — Elodea, the American pond-weed, or some 

 other water-weed, is used for this experiment, as the 

 temperature of water can be kept more uniform than 

 that of air. 



