A POND SCUM 43 



sugars into starch. If we place a drop of iodine solution 

 upon a cell of Spirogyra, each pyrenoid seems to turn blue 

 or blue-black. Careful examination, however, shows that it 

 is not the pyrenoid itself that is colored by the iodine, but a 

 layer, or sometimes two or more layers, of small bodies sur- 

 rounding the pyrenoid. These small bodies are grains of 

 starch. Iodine is a valuable test for starch ; any substance 

 which turns blue or blue-black when in contact with iodine, 

 we may be sure is made of starch or contains starch."^ 



65. Digestion of Starch. — As we have seen, sugar is made 

 only in the sunlight, and when it is manufactured rapidly 

 (as on a bright day) much of it is changed into starch. Dur- 

 ing the night no sugar is made. The cell needs about as much 

 food at night as it does during the day, and so some of the 

 starch that has been stored up during the day is used for food 

 at night. Since the starch itself is not soluble, it must be 

 changed into a soluble form before it can be used — that is, 

 it must be digested. Spirogj'ra digests starch by means of 

 certain enzyms which it produces and which change the 

 starch back into a sugar. 



66. Other Food Substances. — Carbohydrates are made 

 up of carbon, hydrogen, and oxygen. In addition to these 

 elements, living matter contains nitrogen, sulphur, and phos- 

 phorus, as well as smaller amounts of several other elements. 

 The elements that are not present in carbohydrates are ob- 

 tained by the plant in the form of simple salts — largely ni- 

 trates, sulphates, and phosphates — which are present in small 

 quantities in the water in which Spirogyra lives. Out of car- 

 bohydrates and some of these absorbed salts, the ceU builds 

 up a class of very complex compounds known as proteins. Pro- 

 teins are always present in living cells, and up to the present, 

 time they have never been made outside of living cells. 



' Some other substances, very similar in chemical composition to starch, are 

 also turned blue or violet by iodine ; but starch is the only substance so affected 

 that we are likely to meet in everyday life. 



