=Geotropism.=--Roots turn toward the earth, even if the seed is planted with the micropyle up. This phenomenon is called =positive geotropism=. Stems grow away from the earth. This is =negative geotropism=. [Illustration: FIG. 50.--THE GRASP OF A PLANT ON THE PARTICLES OF EARTH. A grass plant pulled in a garden.] [Illustration: FIG. 51.--PLANT GROWING IN INVERTED POT.] [Illustration: FIG. 52.--HOLES IN SOIL MADE BY ROOTS, now decayed. Somewhat magnified.] SUGGESTIONS (Chaps. VII and VIII).--=25.= _Tests for food._ Examine a number of roots, including several fleshy roots, for the presence of food material, making the tests used on seeds. =26.= _Study of root-hairs._ Carefully germinate radish, turnip, cabbage, or other seed, so that no delicate parts of the root will be injured. For this purpose, place a few seeds in packing-moss or in the folds of thick cloth or of blotting paper, being careful to keep them moist and warm. In a few days the seed has germinated, and the root has grown an inch or two long. Notice that, except at a distance of about a quarter of an inch behind the tip, the root is covered with minute hairs (Fig. 44). They are actually hairs; that is, root-hairs. Touch them and they collapse, they are so delicate. Dip one of the plants in water, and when removed the hairs are not to be seen. The water mats them together along the root and they are no longer evident. Root-hairs are usually destroyed when a plant is pulled out of the soil, be it done ever so carefully. They cling to the minute particles of soil (Fig. 46). The hairs show best against a dark background. =27.= On some of the blotting papers, sprinkle sand; observe how the root-hairs cling to the grains. Observe how they are flattened when they come in contact with grains of sand. =28.= _Root hold of plant._ The pupil should also study the root hold. Let him carefully pull up a plant. If a plant grow alongside a fence or other rigid object, he may test the root hold by securing a string to the plant, letting the string hang over the fence, and then adding weights to the string. Will a stake of similar size to the plant and extending no deeper in the ground have such firm hold on the soil? What holds the ball of earth in Fig. 50? =29.= _Roots exert pressure._ Place a strong bulb of hyacinth or daffodil on firm-packed earth in a pot; cover the bulb nearly to the top with loose earth; place in a cool cellar; after some days or weeks, note that the bulb has been raised out of the earth by the forming roots. All roots exert pressure on the soil as they grow. Explain. =30.= _Response of roots and stems to the force of gravity, or geotropism._ Plant a fast-growing seedling in a pot so that the plumule extends through the drain hole and suspend the pot with mouth up (_i.e._ in the usual position). Or use a pot in which a plant is already growing,