224 Missouri Agr. Exp. Sta. Research Bulletin No. 8 



bly hardier than when kept at a temperature of 20 C, while more 

 resistant plants were considerably more hardy when kept at a low 

 temperature. Fisher^ found that it required a lower temperature 

 to change the nature of colloids like starch paste that had been kept 

 at low temperatures than to change the nature of colloids kept 

 at a high temperature. 



Relative Hardiness of Different Tissues at Different Seasons 

 of the Year. When trees are in a rapidly growing condition, appar- 

 ently the most tender part of the wood tissue is the cambium and the 

 young cortex, and sap wood cells. However, in winter after the 

 wood has reached its greatest maturity, this is not the case. In fact 

 when severe cold comes, the first tissue to kill seems to be the pith 

 in the case of young twigs, and there will be browning in the sap 

 wood and part of the cortex. In case of the cortex the browning is 

 generally worse in the outer or older cells. This was observed by 

 Eustace^ on peach trees following the winter of 1903-04. We have 

 often observed the same in artificial freezings we have made, as well 

 as on peach trees badly injured in winter. Peach trees so injured 

 that the sap wood seemed practically all browned have, under favora- 

 ble conditions, had the cambium form new layers of sap wood sur- 

 rounding this wood. This injured wood soon becomes entirely dead 

 and the tree depends on the new sap wood formed for conductive 

 tissue. We have also observed dead areas of bark following the win- 

 ter of 1911-12 when underneath there was healthy new bark and 

 healthy cambium. In these cases, at least, the cortex was more 

 tender than the cambium. 



The fruit buds of the peach in late summer during the grow- 

 ing season are generally about as hardy as the cortex and cambium, 

 or sap wood of the twigs, though perhaps slightly less hardy than the 

 same tissues in older wood; while in winter under normal conditions, 

 at least with peaches, the fruit buds are generally somewhat less 

 hardy than any of the wood tissue, with the possible exception of the 

 pith cells. However, in the case of a cold wave that comes on very grad- 

 ually, say during a period of two or three weeks with a very cold 

 night at the end, some of the buds may survive a temperature low 

 enough to kill the sap wood badly. Thus following the winter of 

 1904-5, when the temperature at Columbia went to -25 C, after 

 several weeks of very cold weather, nearly all of the peach trees 

 had a few live buds left while the wood was very badly damaged ; and 

 in the case of peaches in New York following the winter of 1903-04, 



iBeitr. BioL der Pfl. Vol. 10, pp. 133-234, 1911. (Bibl. No. 40). 

 2New York (Geneva) Agr. Exp. Sta. Bui. 269, 1905. (Bibl. No. 38). 



