killed in the field at the same time, preferably in the afternoon. Counts of chloroplasts alone can be used as measures of response in plants that produce sugar or oil, while either chloroplasts or starch grains or both may be made the basis in starch-forming leaves. Hesselmann (_l. c._, 379) has employed Sachs’s iodine test as a measure of photosynthesis. This has the advantage of permitting macroscopic examination, but the comparison of the stained leaves can give only a very general idea of the relative photosynthetic activity of two or more ecads. The iodine test is made as follows:[17] fresh leaves are placed for a few minutes in boiling water, and then in 95 per cent alcohol for 2–5 minutes, in order to remove the chlorophyll and other soluble substances. The leaves are placed in the iodine solution for ½–3 hours, or until no further change in color takes place. The strength of the solution is not clearly indicated by Sachs, who says: “I used an alcoholic solution of iodin which is best made by dissolving a large quantity of iodin in strong alcohol and adding to this sufficient distilled water to give the liquid the color of dark beer.” This solution may be approximated by dissolving ⅓ gram of iodin in 100 grams of 30 per cent alcohol. The stained leaves are put in a white porcelain dish filled with distilled water, and the dish placed in the strong diffuse light of a window. The colored leaf stands out sharply against the porcelain, and the degree of coloration, and hence of starch content, is determined by the following table: 1. bright yellow or leather yellow (no starch in the chlorenchym) 2. blackish (very little starch in the chlorenchym) 3. dull black (starch abundant) 4. coal black (starch very abundant) 5. black, with metallic luster (maximum starch-content) _ADAPTATION_