From Botany: an elementary text for schools:
To examine gum-resin: Mount a little of the "milky" juice of the leaf stem of the garden poinsettia (Euphorbia pulcherrima). It is of a creamy consistency. Examination under the microscope shows that it is not white, as it seems to the naked eye. The particles are yellowish or colorless and are insoluble. These particles are gum resin. They have been emulsified by the plant, making the juice appear white.
398. CONTENTS NOT IN SOLUTION.—Starch is the most abundant of the solid products of the cell. Starch grains have a definite form for each group of plants, and groups can be determined by the form of their starch grains. Detection of adulteration of various products containing starch is accomplished by the aid of the microscope. In potato starch the grains are ovate, with a "nucleus" near one end, as shown in Fig. 391. In poinsettia they are dumbbell-shaped, with two nuclei (Fig. 391).
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