4 Winter Wheat Varieties aixl Winti r-KiHiiKj 



as indicated by an increase in the amino-acid content, and, on freezing 

 the sap, by less precipitation of the proteins. He measured the increase 

 in hvdrogen-ion concentration of the sap on cooling, and was able to 

 produce bhe same relative precipitation of proteins by adding equivalent 

 i|uantitie8 of acid. However, the ease of precipitation of proteins by 

 freezing apparently cannot always be taken as an index of hardiness, 

 since Chandler (fi) was uniible to find any difference in this respect between 

 the sap of tender and hardy twigs of fruit trees. Again, it should perhaps 

 1)1' remarked that the fraction of the total proteins present in the ex- 

 pressed sap is rather small, and of this only 31-2 per cent, at most is 

 reported by Harvey as precipitated by freezing the juice of iinliardened 

 cabbage. The extent to whicii this justifies a generalisation with regard 

 to the probable behaviour of the proteins within the cell may be regarded 

 as problematic. 



There is some evidence that stability of the dormant condition may 

 be an important protective adaptation. Lidforss ('-'6) noted that a suc- 

 cession of warm days caused regeneration of starch from sugar, with 

 an increase in susceptibility to cold. Chandler (6) found some varieties 

 of peaches to have a longer rest period than others and to be started 

 into growth more slowly by warm periods in the winter. Evidently 

 varieties which can maintain continuous dormancy during the danger 

 period must have a distinct advantage. 



Our present concept of the causes of winter-killing nuiy be briefly 

 summarised. Without doubt, the ultimate cause of death by freezing 

 must be the disorganisation of the protoplasm. Irreversible coagulation 

 or precipitation of the colloidal protein constituents may be caused by 

 increase in concentration of electrolytes in the cell sap on withdrawal of 

 water, or by increase in acidity, or by both factors acting together. The 

 critical minimum temperature necessary to bring this about must be 

 profoundly modified by rate of cooliug, especially if this be slow enough 

 to give time for the hardening process, and by the presence of substances 

 which protect the proteins from precipitation. Split ling of the proteins 

 during hardening may be a protective adaptation. Since the funda- 

 mental feature of the disturbance produced by freezing is withdrawal 

 of water from the cell, intracelhilnr adaptations to resist desiccation 

 must be of prime importance. 



