Frost-cracks.— These usually occur as the result of rapid freezing and shrinking of the outer tissues of trees, especially while the inner tissues are relatively warm. After the cold pe riod is over and warmer weather comes such cracks close up.
Frost This name is applied to wounds that are prevented from healing through the action of late frost in killing the callus rings as they develop. The killing of roots and the lifting of plants from the soil by freezing are familiar phenomena to all gardeners, but fur ther consideration of this subject cannot be undertaken here.
Scorching or Sometimes in hot, dry weather smooth-barked trees and shrubs may have portions of their tissue exposed to strong sunlight actually killed by the high tem perature. In light-colored, sandy soils the re flection of heat from the sand often kills the lower leaves of low-growing trees, shrubs and especially of herbaceous plants. The reflection of sun heat from the south or west side of buildings, etc., may also cause similar injury. The direct injury in such cases is often, how ever, confused with the effects of excessive transpiration. The burning effect of hot, dry winds is largely due to the excessive transpira tion they induce. The burning is especially se vere when there is an insufficiency of soil moisture as the result of drouth or lack of proper cultivation.
Sudden Variations of already indicated, the injury resulting from changes of temperature above the maximum or below the minimum for a species or individual depends in large measure upon the rapidity of the change. If the change is slow, even the individual plant may adjust its vital processes to temperatures which would have resulted in death had they been sudden. Some of the alga, for example, live in hot springs at a temperature of 93° C., while the original stock from which these forms were derived lives in water rarely exceeding 40° C. A transfer of individuals from 40° to 93° would result in almost instant death, while by slow, gradual change they could be accustomed to this ex treme temperature. For flowering plants (Phanerogams) the possible maximum is much lower, 50° being the extreme limit, while the average limit is from 37° to 46°.
Sudden variations well within the fatal lim its, however, are productive of pathological changes. A rose, tomato, violet or almost any other plant grown at a cool temperature (8° to 15° C.), all other conditions remaining the same and then suddenly exposed to 24° to 27° C., will turn yellowish, become weak and spin
dling and very sensitive to insects and parasitic diseases. The same plants, started at 24° to 27° and growing normally at that temperature, if suddenly dropped to 15° or 8° become yellowish and weak and very sensitive to parasite enemies. In either case the change from one temperature to the other could be made without serious injury if made gradually and slowly. For a general discussion of this subject with refer ences to literature, consult Davenport's 'Ex perimental Morphology.) Resting or Dormant resting or dormant period is more or less well defined in many perennial plants. In northern and tem perate latitudes this period coincides with cold weather and has developed in response to this constantly recurring seasonal change. In other cases in hot climates a partial rest may occur during the dry season. In many cases this period of rest has come to be a necessity to the normal and healthy development of the plant. Detmer and Mfiller-Thurgau have shown in the case of potato tubers and many dormant buds that in the fall and early winter the tissues contain only enough sugar to meet the demands of respiration of the dormant cells, not enough to stimulate growth. Miiller-Thurgau has dem onstrated that exposure of tubers dug in August to 0° C. for a few weeks causes an accumula tion of sugar sufficient to start normal germina tion when the cooled tubers are placed under favorable conditions. The cold reduces the amount of sugar used in respiration and prob ably also causes an increase of diastase, the enzyme that converts starch into sugar. The same has been shown to be true also in the case of the dormant buds of many trees and shrubs. Exposure of dormant buds, bulbs, tubers, etc., to sulphuric ether for several days has a similar effect to that produced by exposure to cold. In the case of plants that have their dormant or resting period in dry weather there is an accu mulation and modification of reserve foods pre paratory to the next season of growth. Buds, bulbs and tubers may often be forced to grow before they have accumulated sufficient reserve food in the proper form, but the result is usually a weak plant very subject to disease. A good example of this may be seen in the attempt that is often made to force bulbs that are immature and that have not had their normal period of rest or have been forced the previous season. Variegation or mosaic disease is also often pro duced in this way.