HORTICULTURE. 43 



periods, breeding varieties hardy under Missouri conditions, killing of apples, 

 and killing of cherries and plums. 



The most commonlj' accepted theory dealing with the freezing to death of 

 plant tissues seems to be that killing from cold is due to the withdrawal of 

 water from the protoplasm. Results of many investigations show that the water 

 generally moves out of the cells to form ice crystals in the intercellular spaces. 

 The amount of water loss necessaiy to result in death varies with different 

 plants and different tissues. Experimental data secured by the author lead 

 him to conclude that freezing to death can not be attributed to precipitation of 

 proteids, as claimed by some investigators. 



The author's experiments with seedlings of various plants and with twigs 

 bearing small apples, peaches, and cherries show that for plant tissues that 

 kill at relatively high temperature the killing temperature is reduced whenever 

 the sap density or molar concentration of the sap of the tissue is increased. 

 In these studies increases in sap density were bi'ought about by using minei'al 

 salt solutions and. also by watering the seedlings sparingly. When sap density 

 was reduced by shading the plants the amount of killed tissues at a given 

 temperature was increased. 



Attempts to increase the sap density of winter peach wood and buds by fer- 

 tilizing peach plats with potassium chlorid were unsuccessful. Examinations 

 made in winter, spring, and summer failed to show any difference in sap density 

 between the plats fertilized with potash and those receiving no potash. Like- 

 wise the action of sirring frost on the bloom and of winter cold on the buds was 

 not influenced by the application of potash to the soil. 



Practically all plants used in freezing experiments in this work were tested 

 with reference to the effect of rapid thawing. The results show that in addition 

 to ripe apples, i)ears. and the leaves of Agave americana observed by Miiller- 

 Thurgau and Molisch, leaves of lettuce kill at slightly lower temperature if 

 they are thawed slowly than if thawed rapidly. In the case of all other tissues 

 tested, either by the author or by others, however, including unripe apples and 

 pears, there is no indication that the rate of thawing has anything to do with 

 the amount of killing at a given temperature. Rapid wilting of tissue has not 

 generally increased the resistance of plants to low temperature over that of 

 unwilted tissue with a dry surface. Tissue with a wet surface killed worse at 

 a given temperature than did tissue with no moisture on the surface. Slow 

 wilting or partial withholding of water through a long period was found to 

 increase the resistance of tissue to low temperature. In case of hardy winter 

 buds and wood a rapid decline in temperature greatly increased the severity 

 of injury from a given low temperature. Previous exposure of plant tissue to 

 low temperature above that at which the tissue kills seems to increase its 

 resistance to low temperature. 



As between different plants there appears to be no constant relation between 

 the rate of growth of plant tissue and resistance to low temperature. Young 

 leaves of fruit trees kill at a higher temperature than old mature leaA^es, 

 whereas the young leaves of lettuce withstand a lower temperature than do the 

 older leaves. With fruit trees the most important feature affecting the hardiness 

 of plant tissue is maturity. Maturity in the case of cambium may be intimately 

 associated with the process of drying out. With the cortex, however, there is 

 little difference between the moisture content of unfrozen cortex in seasons 

 when it is very tender and seasons when it is hardy. The wood at the base of 

 the trunk and at the crotches of all rapidly growing branches seems to reach 

 a condition of maturity in early winter more slowly than do most other tissues. 

 Of the tissues above ground during periods when most complete maturity is 

 reached the most tender parts are the pith cells and the fruit buds. In periods 



