28. Root hold of plant. The pupil should also study the root hold. Let him carefully pull up a plant. If a plant grows 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.
Fig. 51.—Plant growing in Inverted Pot.
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, 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?
Fig. 52.—Holes in Soil made by Roots, now decayed. Somewhat magnified.
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 into it so as to pierce both shell and membrane; or use a short glass tube, first scraping the shell thin with a knife and then boring through it with the tube. Now set the egg upon the mouth of a pickle jar nearly full of water, so that the large end with the exposed membrane is beneath the water. After several hours, observe the tube on top of the egg to see whether the water has forced its way into the egg and increased its volume so that part of its contents are forced up into the tube. If no tube is at hand, see whether the contents are forced through the hole which has been made in the small end of the egg. Explain how the law of osmosis is verified by your result. If the eggshell contained only the membrane, would water rise into it? If there were no water in the bottle, would the egg-white pass down into the bottle? 42. The region of most rapid growth. The pupil should make marks with waterproof ink (as Higgins’ ink or indelible marking ink) on any soft growing roots. Place seeds of bean, radish, or cabbage between layers of blotting paper or thick cloth. Keep them damp and warm. When stem and root have grown an inch and a half long each, with waterproof ink mark spaces exactly one-quarter inch apart (Figs. [48], [49]). Keep the plantlets moist for a day or two, and it will be found that on the stem some or all of the marks are more than one-quarter inch apart; on the root the marks have not separated. The root has grown beyond the last mark.
CHAPTER IX
THE STEM—KINDS AND FORMS; PRUNING
The Stem System.—The stem of a plant is the part that bears the buds, leaves, flowers, and fruits. Its office is to hold these parts up to the light and the air; and through its tissues the various food materials and the life-giving fluids are distributed to the growing and working parts.
The entire mass or fabric of stems of any plant is called its stem system. It comprises the trunk, branches, and twigs, but not the stalks of leaves and flowers that die and fall away. The stem system may be herbaceous or woody, annual, biennial, or perennial; and it may assume many sizes and shapes.