Physiologie der Zellen, Gewebe und Organe. 133 



to death is applied here only to a very specific set of phenomena. With all 

 plant tissues, when a certain temperature is reached very shortly after thawing, 

 it will be found that the tissue has taken on a brown, water-soaked appearance, 

 and evaporation from that tissue is much more rapid than from living tissue. 

 These are characteristics of plant tissue frozen to death. In the experiments 

 described in this bulletin, the killing temperature of plant tissue that kills at 

 relatively high temperature has been reduced whenever the sap density of the 

 tissue has been increased. In the case of all except ripe apples and pears and 

 leaves of lettuce 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. However, 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 in- 

 creased 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. There seems to be no constant relation 

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

 Previous exposure to low temperature above that at which the tissue kills seems 

 to increase the resistance of tissue to low temperature. The most important 

 feature affecting the hardiness of plant tissue is maturity, that is, the condition 

 of resistance that the plants reach during the winter dormant period. Maturity 

 in the case of cambium may be intimately associated with the process of drying 

 out. However, this cannot be true at least of cortex of winter twigs. 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 

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

 condition of maturity in early winter more slowly than does most other tissue. 

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

 reached, the most tender parts are the pith cells and the fruit buts. During 

 periods of rapid growth there is little difference in hardiness of the different 

 tissues. The tissue which is most tender at all seasons of the year is the root. 

 There is much less difference, however, in the killing temperature of roots in 

 summer and winter than between the killing temperature of twigs or other wood 

 in summer and winter. That part of the root System nearest the surface and the 

 largest, oldest roots are more resistant to cold than are small roots further from 

 the surface. Pollen of the apple will withstand much lower temperatures than 

 will any other tissue of the flower when in füll bloom. Scales of peach buds do 

 not serve to protect them from low temperature. Buds frozen in the laboratory 

 with the scales removed were slightly more resistant to low temperature than 

 were buds with the scales not removed. The killing of wood of peach trees from 

 i'reezing is one of the most important determining factors in peach growing. 

 Little can be done to influence the amount of killing except to have the trees 

 start into winter in proper condition of maturity. The hardiness of peach buds 

 when in fully dormant condition seems to be greatly increased by continuous low 

 temperature preceding the date at which the temperature goes low enough to kill. 

 This capacity to withstand low temperatures seems likely to be due to the slow 

 fall in temperature under such conditions rather than to hardiness developed as 

 the result of exposure to low temperature. So far the investigations indicate 

 that early varieties of peaches are not started into growth more readily by warm 

 periods of winter than are later varieties. Some of the very early varieties of 



