=138. Green parts of plants form starch when exposed to light.=—Thus we find that in the case of all the green plants we have examined, starch is present in the green cells of those which have been standing for some time in the sunlight where the process of the absorption of CO₂ and the giving off of oxygen can go on, and that in the case of plants grown in the dark, or in leaves of plants which have stood for some time in the dark, starch is absent. We reason from this that starch is the product of the chemical change which takes place in the green cells under these conditions. The CO₂ which is absorbed by the plant mixes with the water (H₂O) in the cell and immediately forms carbonic acid. The chlorophyll in the leaf absorbs radiant energy from the sun which splits up the carbonic acid, and its elements then are put together into a more complex compound, starch. This process of putting together the elements of an organic compound is a _synthesis_, or a _synthetic assimilation_, since it is done by the living plant. It is therefore a synthetic assimilation of carbon dioxide. Since the sunlight supplies the energy it is also called _photosynthesis_, or _photosynthetic assimilation_. We can also say carbon dioxide assimilation, or CO₂ assimilation (see paragraph on assimilation at close of Chapter 10). =139. Starch is formed only in the green parts of variegated leaves.=—If we test for starch in variegated leaves like the leaf of a coleus plant, we shall have an interesting demonstration of the fact that the green parts of plants only form starch. We may take a leaf which is partly green and partly white, from a plant which has been standing for some time in bright light. Fig. 68 is from a photograph of such a leaf. We should first boil it in alcohol to remove the green color. Now immerse it in the potassium iodide of iodine solution for a short time. The parts which were formerly green are now dark blue or nearly black, showing the presence of starch in those portions of the leaf, while the white part of the leaf is still uncolored. This is well shown in fig. 69, which is from a photograph of another coleus leaf treated with the iodine solution. 3. Chlorophyll and the Formation of Starch. =140.= In our experiments thus far in treating of the absorption of carbon dioxide and the evolution of oxygen, with the accompanying formation of starch, we have used green plants. =141. Fungi cannot form starch.=—If we should extend our experiments to the fungi, which lack the green color so characteristic of the majority of plants, we should find that photosynthesis does not take place even though the plants are exposed to direct sunlight. These plants cannot then form starch, but obtain carbohydrates for food from other sources.