Sun-Burning
A term applied to injuries of various parts of plants, attributed, with more or less reason, to exposure to the rays of the sun in confined spaces, or where the heat is concentrated through glass or water, or by reflection from walls or other objects. Probably, both heat-rays and light-rays are concerned in the result; but it is impossible to determine the share due to each. A very rapid rise of temperature, after severe cold, is also apt to produce results included under the general name of Sun-burning. Most plants, if exposed for some time to a temperature between 105deg. and 125deg. Fahr., are killed; but certain fleshy subjects, particularly species from tropical deserts, are able to resist even a higher temperature than 125deg. When plants that have been in a greenhouse all the winter are first put out of doors, in spring, the leaves often become brown and look scorched. These organs may die and wither, or may only become red or brown, and afterwards reassume their natural green colour. Plants do not usually suffer serious injury from this cause, though often checked in their growth for a time.
In ill-ventilated glasshouses, it often occurs that the leaves of many of the plants show round, pale, withered spots. Observation teaches that these follow the presence on the leaves, during sunshine, of drops of water; and it has been suggested that the drops act like miniature burning-glasses, focussing the rays upon the spots below them, and destroying the protoplasm by the excess of light and heat so caused. Inequalities in the glass of greenhouses are believed to give rise to similar injuries. Whatever may be the cause, experience has proved that thorough ventilation is the best preventive, and that, if this is attended to, the evil will be much diminished, if not wholly cured.
Sun-burning and Sunstroke are terms applied to injuries of the stems of trees, consisting sometimes in the bark dying, and separating from considerable surfaces, or in long strips, on the side most exposed to the sun's rays. Sunstroke is apt to occur in trees exposed by the removal of others or of undergrowth, or it may follow the erection of walls or other reflectors of heat in the immediate neighbourhood of the stems. It injures both fruit-trees (especially Peaches) and forest-trees when suddenly exposed. The cause of injury is the death of the newly-formed cells of the cambium layer by exposure to excessive heat. In trees suddenly exposed to variations in temperature greater than they had previously to bear, the bark is not sufficiently developed to protect the cambium from injury, and the result is as just described.
Splitting of the bark is apt to occur in spring, when trees have been exposed for some time to keen frost, followed by a sudden rise of temperature. The injury is due to unequal expansion of the bark and the wood. Probably, it is more often caused by the preceding frost than by the warmth, the thaw only bringing to view the harm already done. But that Splitting is, to some extent, connected with the sun's warmth is shown by its greater frequency on the side of the trunk that receives most of the sun's rays. Both Sun-burning and Splitting are far commoner on the Continent than in Britain, where the extremes of heat and cold are seldom such as to give rise to either. The best preventive of both is to protect the trunks and branches that may be in danger, by some simple means of shelter, such as wrapping them in straw, or coiling a straw-rope around them.
A few years ago, Dr. H. Muller, of Thurgau, called attention to the loss of grapes that occurs in various places in Germany when cold, damp weather is suddenly followed by bright, warm, sunny days. In the unripe clusters that are exposed to the direct rays of the sun, many of the grapes become pale, and then shrivel and turn brown. Sometimes, the stalks of the clusters turn brown before the grapes show signs of injury, but the latter soon shrivel and die when the stalks are killed. Experiments show that the cause of injury is excessive heat, and that similar results follow when clusters are exposed to artificial heat as great as that to which they are subjected under natural conditions, i.e., between 105deg. and 115deg. Fahr. The risk of injury is greater in proportion as the fruit is more juicy. The moister the atmosphere, the less must be the evaporation from the fruit; hence, in moist weather, evaporation cannot act as a means of keeping down the temperature, and this probably explains how the harm done is greater after a continuance of wet weather, as the air is then nearly saturated with moisture. The risk is much less in England than in Germany, as the temperature is seldom so high in our islands as to do injury. Prevention may be secured by any method of shading the clusters. The best protection is that afforded by the leaves of the Vines themselves; hence, the removal of the leaves to hasten ripening is seldom to be recommended.
Though not strictly included under Sun-burning, a few remarks may be added on the harm done by dry, windy weather to many plants that grow in moist soil. If exposed for some days to dry, warm winds, the leaves of such plants wither and die, becoming so brittle as to crumble into dust when rubbed in the hand. Under similar conditions, the same species of plants growing in drier ground may scarcely be injured. The cause is as follows: The damper the localities in which plants grow, the more are all their green parts adapted for rapid evaporation. While the evaporation is balanced by the amount of water absorbed by the roots, and carried up by the stems, growth goes on rapidly, and the plants grow luxuriantly, unless the mineral matters absorbed with the water accumulate to a hurtful degree, when the plant will become weak and sickly. But in dry, warm weather, the loss by evaporation exceeds the amount that can be supplied to the leaves; hence, they wither, and dry up. Those that suffer most are the mature leaves in the active discharge of their functions, the older and younger ones being far less severely injured. The only remedial means applicable is to shelter the choicer plants from wind and sunshine as much as possible, and to syringe their leaves occasionally. Watering the soil is of no use, as it is already too moist. On such ground, drainage is the most efficient means of preventing injury to the plants from drought. SURFACE ATERPILLARS, or SURFACE GRUBS. The larvae of several species of Moths, chiefly Triphaena pronuba, Agrotis segetum, and A. exclamationis. The name is given to these larvae because of their habit of living just below the surface of the soil during the day, coming out to feed at night on low plants and herbs. They are much in the habit of gnawing through the stems of herbaceous plants below the soil, just where the stems join the roots, and thus cause the plants to wither without any evident reason. In winter, they also frequently eat their way into fleshy tubers (potatoes) and tuberous roots (turnips, &c.), Asters, many weeds, &c., and do considerable damage. In spring, the larvae become pupae in earthen cocoons; and in summer, the moths emerge. Plants suffer most from the larvae in ungenial summers, when growth is slow; and the damage then done permanently injures them. Rains occasionally enable plants to recover, even when they have suffered severely. Further information about these larvae will be found under Noctua; and the best remedies are mentioned under that heading.