The action of frost may result in long splits in the trunks of trees, or in the killing of the ends of the branches, which soon blacken in conse quence. The injured parts should be removed by pruning.
Air.—Every living cell must have a constant supply of oxygen in order to exist. The stem some times suffers by applications of tar which shuts out air. The roots commonly suffer and often are killed by being deprived of air. This happens when the soil is too wet or when a hard crust is allowed to form on the surface. For the same reason, paving sidewalks or covering the roots deeply with soil may be injurious. When the surface of the soil is loose and sufficiently dry, a circulation of air is kept up within the soil by constant changes in barometric pressure. When this is prevented the soil becomes sour and unfit for plants, and the chemical processes that make food available to the plant are checked. Roots may grow in running water, which constantly renews the supply of dis solved air. Some roots can live in mud, but they are supplied with air by way of the leaves and large air-passages in the stem ; they are specially adapted to such environment. It is therefore of the utmost importance to maintain a loose, open tex ture of the soil by proper tillage, to ensure the health and vigor of most agricultural plants.
Wind.—The curiously gnarled and bent appear ance of trees that are daily exposed to strong winds is familiar to all. In many cases all the branches on the windward side are killed. This is due to the drying effect of the wind, which may increase evaporation as much as twenty-fold. The mere mechanical effect of strong prevailing winds is often very marked. It is common to see trees with the tips of the branches permanently turned leeward, or with the heavy growth all on one side. Trees on mountain tops and near sea-coasts are often weirdly picturesque, from wind action.
The effect of wind in drying fruit blossoms is well known, as well as the mechanical damage to branches laden with ice and snow. For this reason
the planting of windbreaks is often indispensable.
Environment and inheritance.—The facts just mentioned show how readily the plant responds to the influence of environment by altering its struc ture or functions. The way in which it responds is determined in each case by the qualities it has received from its ancestors. The form of the plant, therefore, depends on both these factors.
Some plants are plastic and easily modified by external influences ; others are not so readily affected. The very remarkable alterations pro.. duced by insects, including the various kinds of galls, the "witches' brooms" produced by attacks of fungi, completely altering the habit of the plant, and the "green flowers" due to small insects, make us realize the great possibilities of external influences. The analysis of all these phenomena should enable us eventually to control them.
Literature.
The reader is referred to the following publica tions for further information : Lectures on the Physiology of Plants, J. Sachs ; the two books, Power of Movement in Plants, and The Various Contrivances by Which Orchids are Fertilized by Insects, Charles Darwin ; Text-book of Botany, E. Strasburger and others ; The Physiology of Plants, W. Pfeffer ; Lectures on the Physiology of Plants, S. H. Vines ; Plant Geography, A. F. W. Schimper ; Organography of Plants, K. Goebel ; Comparative Anatomy of the Vegetable Organs of the Phanero gams and Ferns, A. de Bary ; Plant Physiology, Paul Sorauer ; Practical Text-book of Plant Phys iology, D. T. MacDougal ; An Introduction to Vegetable Physiology, J. R. Green ; Text-book of Plant Physiology, G. J. Pierce ; the two books, Disease in Plants, and The Oak, H. M. Ward ; Natural History of Plants, Anton Kerner ; The Great World's Farm, S. Gaye ; The Soil, F. H. King. There are many good school and college texts that will aid the general reader.