=155. Vaucheria.=—The result of carbon dioxide assimilation in the threads of Vaucheria is not clearly understood. Starch is absent or difficult to find in all except a few species, while oil globules are present in most species. These oil globules are spherical, colorless, globose and highly refringent. Often small ones are seen lying against chlorophyll bodies. Oil is a _hydrocarbon_ (containing C, H, and O, but the H and O are in different proportions from what they are in H₂O) and until recently it was supposed that this oil in Vaucheria was the direct result of photosynthesis. But the oil does not disappear when the plant is kept for a long time in the dark, which seems to show that it is not the direct product of carbon dioxide assimilation, and indicates that it comes either from a temporary starch body or from glucose. Schimper found glucose in several species of Vaucheria, and Waltz says that some starch is present in Vaucheria sericea, while in V. tuberosa starch is abundant and replaces the oil. To test for oil bodies in Vaucheria treat the threads with weak osmic acid, or allow them to stand for twenty-four hours in Fleming’s solution (which contains osmic acid). Mount some threads and examine with microscope. The oil globules are stained black. 2. Sugar, and Digestion of Starch.[9] =156.= It is probable that some form of sugar is always produced as the result of photosynthesis. The sugar thus formed may be stored as such or changed to starch. In general it may be said that sugar is most common in the green parts of monocotyledonous plants, while starch is most frequent in dicotyledons. Plant sugars are of three general kinds: cane sugar abundant in the sugar cane, sugar beet, sugar maple, etc.; glucose and fruit sugar, found in the fruits of a majority of plants, and abundant in some, as in apples, pears, grapes, etc.; and maltose, a variety produced in germinating seeds, as in malted barley. =157. Test for sugar.=—A very pretty experiment maybe made by taking two test tubes, placing in one a solution of commercial grape sugar (glucose), in the other one of granulated cane sugar, and adding to each a few drops of Fehling’s solution.[10] After these tubes have stood in a warm place for half an hour, it will be found that a bright orange brown or cinnabar-colored precipitate of copper and cuprous oxide has formed in the tube containing grape sugar, while the other solution is unchanged. Grape sugar or glucose, therefore, reduces Fehling’s solution, while cane sugar as such has no effect upon it. Cane sugar may be changed or converted to glucose by being boiled for a short time with a dilute acid, or by adding Fehling’s solution to the sugar solution and boiling. In the latter case the change is brought about by the alkali and the precipitate of copper and cuprous oxide forms.