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, cover with cloth or wire gauze to prevent the soil from falling, and suspend the pot in an inverted position (Fig. 51). Notice the behaviour of the stem, and after a few days remove the soil and observe the position of the root. =31.= If a pot is laid on one side, and changed every two days and laid on its opposite side, the effect on the root and stem will be interesting. =32.= If a fleshy root is planted wrong end up, what is the result? Try it with pieces of horse-radish root. =33.= By planting radishes on a slowly revolving wheel the effect of gravity may be neutralized. =34.= _Region of root most sensitive to gravity._ Lay on its side a pot containing a growing plant. After it has grown a few days, wash away the earth surrounding the roots. Which turned downward most decidedly, the tip of root or the upper part? =35.= _Soil texture._ Carefully turn up soil in a rich garden or field so that you have unbroken lumps as large as a hen’s egg. Then break these lumps apart carefully with the fingers and determine whether there are any traces or remains of roots (Fig. 52). Are there any pores, holes, or channels made by roots? Are the roots in them still living? =36.= Compare another lump from a clay bank or pile where no plants have been growing. Is there any difference in texture? =37.= Grind up this clay lump very fine, put it in a saucer, cover with water, and set in the sun. After a time it will have the appearance shown in the lower saucer in Fig. 43. Compare this with mellow garden soil. In which will plants grow best, even if the plant-food were the same in both? Why? =38.= _To test the effect of moisture_ on the plant, let a plant in a pot or box dry out till it wilts; then add water and note the rapidity with which it recovers. Vary the experiment in quantity of water applied. Does the plant call for water sooner when it stands in a sunny window than when in a cool shady place? Prove it. =39.= Immerse a potted plant above the rim of the pot in a pail of water and let it remain there. What is the consequence? Why? =40.= _To test the effect of temperature on roots._ Put one pot in a dish of ice water, and another in a dish of warm water, and keep them in a warm room. In a short time notice how stiff and vigorous is the one whose roots are warm, whereas the other may show signs of wilting. =41.= _The process of osmosis._ Chip away the shell from the large end of an egg so as to expose the uninjured membrane beneath for an area about as large as a ten cent piece. With sealing-wax, chewing-gum, or paste, stick a quill about three inches long to the smaller end of the egg. After the tube is in place, run a hat pin