Certain varieties of peaches and plums which survive Northern winters without injury, when planted in the South often winter-kill. Ex tended warm spells in the latter part of winter, and the warming effects of the sun on the south and southwest sides of the trunk start the ac tivity of the cambium, which is subsequently injured or killed by freezing, the same as are young buds pushing out too early in the spring under the influence of warm days and when caught by the late frosts.
Varieties that start into growth readily under such conditions may be completely winter-killed if the conditions are such as to stimulate a gen eral premature activity. If only the tissues warmed by the direct action of the sun are in volved, patches of various sizes may be killed. These usually start near the surface of the ground on the southwest side of the tree and extend up the trunk to the limbs, varying in width from a few inches to the whole south and southwest side.
Winter Sun Scald.— This term is usually applied to cases like those just described where smaller areas on the south or southwest side of trees are killed during winter by the successive warming and freezing of the tissues. This form of injury is common in the North and West. Considerable protection may be afforded in all such cases by coating the tree with whitewash or protecting it from the direct action of the sun in some way.
Winter Desiccation.— Evergreen trees and shrubs are often killed or greatly injured during periods of bright warm weather in the winter by rapid evaporation of water from the leaves while the ground and roots are frozen and un able to replace what is lost.
Another form of winter desiccation is through the direct action of freezing, where, by intense cold, the water is drawn so completely from the protoplasm into the cell walls or in tercellular spaces in the formation of ice crys tals that the cells are unable to reabsorb water on thawing. This effect of freezing is similar in many respects to that of drouth. Water is withdrawn from the protoplasm to a greater or less extent, according to the duration and severity of the cold and the retaining power of the protoplasm. If the withdrawal has not been too great and thawing process is slow so that the cells can gradually reabsorb what was given up in freezing, the plant may suffer no harm.
If on the other hand, the tissues are thawed rapidly, the air is driven out of the intercellular spaces and the cells may die for want of oxygen or the water may be lost by evaporation before it can be reabsorbed. Frozen plants should, therefore, be thawed slowly and protected from evaporation by sprinkling with cold water or shading. While most cases of injury from freezing are due to the conditions just de scribed, there is another class of cases where the injury is apparently due to structural and chemical changes in the protoplasm induced by the cold.
Cold Rigor.—As already pointed out, the sensitiveness of protoplasm of various plants and at various stages of growth differs remark ably, but in all cases during vegetative activity there is a point either a little above or below the freezing point of water where protoplasmic movement and growth cease without the cells being i necessarily frozen or killed. This condi tion s known as cold rigor and is often taken advantage of by florists and gardeners in hold ing their flowers or fruits temporarily in some particular stage of development for a particular market period. Uusually when the temperature is raised the growth continues normally, but occasionally, if the temperature is a little too low and long continued, the plant fails to re,• spond again to a higher temperature and is found to be in a state of permanent rigor or paralysis; though the plant does not die, it makes little or no growth.
Mechanical Separation of In many parenchymatous tissues, leaves, bark, paren chyma, fruits, etc., which can stand considerable freezing without injury to the protoplasm, there is more or less rupture of the cells from each other by the formation of ice in the cell walls or intercellular spaces. In some cases the epi dermal cells are almost completely separated from the rest of the tissue. In other cases, as in apples and bark parenchyma, the individual cells may, in extreme cases, be separated and form a powdery mass. If the separation is complete the cells die after a time. In any case, even a partial separation and breaking of the protoplasmic connections between the cells in terferes with normal development, and may cause the premature death of the whole or parts of the plant by interfering with the movement of food materials, etc.