=136. Iodine used as a test for starch.=—This color reaction which we have obtained in treating the threads with iodine is the well-known reaction, or test, for starch. We have demonstrated then that starch is present in spirogyra threads which have stood in the sunlight with free access to carbon dioxide. If we examine in the same way some threads which have stood in the dark for a few days we obtain no reaction for starch, or at best only a slight reaction. This gives us some evidence that a chemical change does take place during this process (absorption of CO₂ and giving off of oxygen), and that starch is a product of that chemical change. =137. Schimper’s method of testing for the presence of starch.=—Another convenient and quick method of testing for the presence of starch is what is known as Schimper’s method. A strong solution of chloral hydrate is made by taking 8 grams of chloral hydrate for every 5_cc_ of water. To this solution is added a little of an alcoholic tincture of iodine. The threads of spirogyra may be placed directly in this solution, and in a few moments mounted in water on the glass slip and examined with the microscope. The reaction is strong and easily seen. We should also examine the leaves of elodea, or one of the higher green plants which has been for some time in the sunlight. We may use here Schimper’s method by placing the leaves directly in the solution of chloral hydrate and iodine. The leaves are made transparent by the chloral hydrate so that the starch reaction from the iodine is easily detected. The following is a convenient and safe method of extracting chlorophyll from leaves. Fill a large pan, preferably a dishpan, half full of hot water. This may be kept hot by a small flame. On the water float an evaporating dish partly filled with alcohol. The leaves should be first immersed in the hot water for several minutes, then placed in the alcohol, which will quickly remove the chlorophyll. Now immerse the leaves in the iodine solution. [Illustration: Fig. 68. Leaf of coleus showing green and white areas, before treatment with iodine.] [Illustration: Fig. 69. Similar leaf treated with iodine, the starch reaction only showing where the leaf was green.]