Practically all the chlorophylla ceous plants are dependent upon sunlight for normal development and activity. Every in dividual and every species is adapted or attuned to a particular range of light intensity and suffers injury if the intensity becomes too great or too small, or if the change from one in tensity to another is too sudden.
Injury from Strong Many bac teria and fungi, for example, Bacillus anthracis and Botrytis cinerea, grow best in darkness or very weak light, but are killed in a short time by exposure to sunlight or to artificial light strong in the blue-violet rays. It is this por tion of the spectrum that causes the chemical disorganization of the protoplasm or rather its destruction by oxidation as in the absence of oxygen during exposure no change takes place. The same is true of the more highly developed chlorophyllaceous algae and shade plants --- too strong light not only destroys the chlorophyll by oxidation induced by the blue-violet rays, but the protoplasm itself may finally be thus killed or chemically disorganized. This is also well shown in the injurious effect of the ultra violet rays in the naked electric arc light. A glass globe which cuts off these rays prevents the injury. This pathological condition in vary ing degrees of intensity is produced not only in shade plants when exposed to direct sunlight, but also in other plants when grown in the shade and suddenly exposed to strong light. It is also common when an extended period of cloudy or foggy weather in the spring is sud denly followed by bright, clear days. Under such conditions there is not only a partial de struction of the chlorophyll, but an excessive loss of water from the poorly cuticularized leaves. Such leaves are also unable to close their stomata quickly, which is another reason for their suffering from excessive loss of water. The action of light in promoting evaporation or transpiration is mainly by raising the tempera ture of the tissues above that of the surround ing air. When the air temperature is very high the increased temperature of the tissues of plants exposed to strong sunlight is often suffi cient in itself to cause injury or death.
Injury from Weak While too strong light cannot be endured by any plant, lack of light is injurious only to cholorophyl laceous. plants. When the light intensity is re duced below the minimum for any of these plants the synthesis of carbon and nitrogen compounds is reduced accordingly until in very weak light or darkness the plant can make no use whatever of the carbondioxide of the air. That the plant will starve in proportion to the reduction in photosynthetic activity is evident.
That electrical conditions of the earth and atmosphere and of the plant have an important relation to the physiological con dition of the latter has been demonstrated in many cases, but the full significance of this re lation is not yet understood. Plants vary in sensitiveness to electric currents in much the same way that they do to light or heat. A current above the maximum for a plant causes the disorganization and death of the protoplasm. This is often accomplished by a comparatively weak direct or alternating current, but is per haps most familiar in cases of injury or death from currents of high intensity, as from elec tric-light wires or lightning stroke.
Mechanical Finally, the causes of disease in the non-living environ ment, we have to mention mechanical injuries, such as those produced by hail, wind, falling trees, etc. Hail stones often severely cut and tear the foliage of trees and herbaceous plants and may also produce more or less serious wounds in the more resistant tissues. Wind may injure the foliage, break limbs, etc., as may also falling trees. In all these cases, except in the most severe forms, the greatest danger comes from the entrance into the tissues of the wounded plant of parasitic insects and fungi. Large mechanical wounds, however produced, should have antiseptic treatment where practi cable, especially in large and valuable trees. The injured parts should be trimmed to healthy tissue in such a manner as to favor rapid for mation of callus. After a wash of some disin fectant, such as copper sulphate 1 per cent solu tion in water, the wound should be protected by shellac or grafting wax or white lead paint.