Fig. 128.—To show Sap Pressure.
94. Cut a succulent shoot of any plant, thrust the end of it through a hole in a cork, and stand it in a small bottle of water. Invert over this a fruit jar, and observe that a mist soon accumulates on the inside of the glass. In time drops of water form. 95. The experiment may be varied as shown in Fig. [125]. 96. Or, invert the fruit jar over an entire plant, as shown in Fig. [126], taking care to cover the soil with oiled paper or rubber cloth to prevent evaporation from the soil.
Fig. 126.—To illustrate Transpiration.
Fig. 127.—Loss of Water.
97. The test may also be made by placing the pot, properly protected, on balances, and the loss of weight will be noticed (Fig. [127]). 98. Cut a winter twig, seal the severed end with wax, and allow the twig to lie several days. It shrivels. There must be some upward movement of water even in winter, else plants would shrivel and die. 99. To illustrate sap pressure. The upward movement of sap water often takes place under considerable force. The cause of this force, known as root pressure, is not well understood. The pressure varies with different plants and under different conditions. To illustrate: cut off a strong-growing small plant near the ground. By means of a bit of rubber tube attach a glass tube with a bore of approximately the diameter of the stem. Pour in a little water. Observe the rise of the water due to the pressure from below (Fig 128). Some plants yield a large amount of water under a pressure sufficient to raise a column several feet; others force out little, but under considerable pressure (less easily demonstrated). The vital processes (i.e., the life processes). 100. The pupil having studied roots, stems, and leaves, should now be able to describe the main vital functions of plants: what is the root function? stem function? leaf function? 101. What is meant by the “sap”? 102. Where and how does the plant secure its water? oxygen? carbon? hydrogen? nitrogen? sulphur? potassium? calcium? iron? phosphorus? 103. Where is all the starch in the world made? What does a starch-factory establishment do? Where are the real starch factories? 104. In what part of the twenty-four hours do plants grow most rapidly in length? When is food formed and stored most rapidly? 105. Why does corn or cotton turn yellow in a long rainy spell? 106. If stubble, corn stalks, or cotton stalks are burned in the field, is as much plant-food returned to the soil as when they are ploughed under? 107. What process of plants is roughly analogous to perspiration of animals? 108. What part of the organic world uses raw mineral for food? 109. Why is earth banked over celery to blanch it? 110. Is the amount of water transpired equal to the amount absorbed?
Fig. 129.—Before and after Pruning.