=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? =111.= Give some reasons why plants very close to a house may not thrive or may even die. =112.= Why are fruit trees pruned or thinned out as in Fig. 129? _Proper balance between top and root._ =113.= We have learned that the leaf parts and the root parts work together. They may be said to balance each other in activities, the root supplying the top and the top supplying the root (how?). If half the roots were cut from a tree, we should expect to reduce the top also, particularly if the tree is being transplanted. How would you prune a tree or bush that is being transplanted? Fig. 130 may be suggestive. [Illustration: FIG. 130.—AN APPLE TREE, with suggestions as to pruning when it is set in the orchard. At _a_ is shown a pruned top.] [Illustration: FIG. 129.—BEFORE AND AFTER PRUNING.] CHAPTER XIV DEPENDENT PLANTS