116 PRINCIPLES OF PLANT CULTURE 



clear to the cambium, when the bark ceases to adhere, 

 and the tree or branch thus affected dies. In stone fruits, 

 this trouble is often accompanied by a flow of gum. If the 

 coloring of the wood does not extend to the cambium, the 

 tree or branch may survive, but the first season's growth 

 thereafter is generally feeble and the fruit or the seed 

 crop often fails. During the second season, healthy 

 growth may be resumed, but the heart-wood is rarely or 

 never restored to its normal color. Black-heart may re- 

 sult from other causes than cold. 



Other chemical changes result from cold, as the sweeten- 

 ing of potato tubers when chilled, the removal of astrin- 

 gency from the wild grape and persimmon, and the 

 heightening of the flavor of the parsnip. 



193. Splitting of wood. — Tree trunks are often split 

 open by severe freezing, the split or frost crack remaining 

 open until the return of mild weather. This most often 

 occurs in hard-wooded, deep-rooted deciduous trees, as 

 the oak. Frost cracks are due to the fact that upon 

 freezing the water is withdrawn from the walls of the cells 

 and vessels of the woody tissues, in other words, they 

 " freeze dry." This causes them to shrink and so crack 

 open just as they " check " in ordinary drying. Frost 

 cracks close in summer and as a rule heal by callus growth 

 (72) pushing together from the two sides. Severe freezing 

 may again tear this open the next winter. Such successive 

 stimulations to callus growth often give rise to prominent 

 ridges bordering the frost crack and known as frost ribs. 



The splitting down of the main branches of certain 

 varieties of the apple tree appears to be favored by the 

 expansive force of ice in narrow crotches, which retain 

 snow and water. Varieties the branches of which leave 

 the trunk at a wide angle are not subject to this trouble. 



