Large wounds should be protected with a covering of paint, melted wax, or other adhesive and lasting material, to keep out the germs and fungi. A covering of sheet iron or tin may keep out the rain, but it will not exclude the germs of decay; in fact, it may provide the very moist conditions that such germs need for their growth. Deep holes in trees should be treated by having all the decayed parts removed down to the clean wood, the surfaces painted or otherwise sterilized, and the hole filled with wax or cement.
Fig. 66.—A Knot Hole, and the beginning of a hollow trunk.
Stems and roots are living, and they should not be wounded or mutilated unnecessarily. Horses should never be hitched to trees. Supervision should be exercised over persons who run telephone, telegraph, and electric light wires, to see that they do not mutilate trees. Electric light wires and trolley wires, when carelessly strung or improperly insulated, may kill trees (Fig. [67]).
Fig. 67.—Elm Tree killed by a Direct Current from an Electric Railroad System.
Suggestions.—Forms of stems. 43. Are the trunks of trees ever perfectly cylindrical? If not, what may cause the irregularities? Do trunks often grow more on one side than the other? 44. Slit a rapidly growing limb, in spring, with a knife blade, and watch the result during the season. 45. Examine the woodpile, and observe the variations in thickness of the annual rings, and especially of the same ring at different places in the circumference. Cross-sections of horizontal branches are interesting in this connection. 46. Note the enlargement at the base of a branch, and determine whether this enlargement or bulge is larger on long, horizontal limbs than on upright ones. Why does this bulge develop? Does it serve as a brace to the limb, and is it developed as the result of constant strain? 47. Strength of stems. The pupil should observe the fact that a stem has wonderful strength. Compare the proportionate height, diameter, and weight of a grass stem with those of the slenderest tower or steeple. Which has the greater strength? Which the greater height? Which will withstand the most wind? Note that the grass stem will regain its position even if its top is bent to the ground. Note how plants are weighted down after a heavy rain and how they recover themselves. 48. Split a cornstalk and observe how the joints are tied together and braced with fibres. Are there similar fibres in stems of pigweed, cotton, sunflower, hollyhock?
Fig. 68.—Potato. What are roots, and what stems? Has the plant more than one kind of stem? more than two kinds? Explain.