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next. The pith body itself usually does not suffer until later and the older 

 the branch is the less is the injury to the pith body. In the present case, 

 the experiment was carried out toward the middle of May, at which time 

 the storing of starch had already taken place in the pith and bark. The 

 injury to the pith was limited here to a checkered marking of the pith disc, 

 while the contents of some of the cells containing starch had turned brown. 

 Investigation showed that the cytoplasmatic substances, and not the starch 

 grains themselves, were discolored. 



The irregular distribution of cells browned by frost in all tissues can 

 be explained only by the different cell content. Probably cells rich in 

 sugar are the most susceptible. The cytoplasmatic content has suffered 

 even when the cell membrane is still clear. In injuries to the pith crown 

 the narrow, spiral ducts are the first ones to be browned. 



The Frost Line. 



Mention was made in the previous section that the fruit grower usually 

 considers the browned cambial region as an indication of frost injury. 

 This zone is now often termed "frost line." Even unskilled forest workers 

 showed me, as frost hues, the circular zones, setting in after spring frosts, 

 between older annual rings with which we will later become better ac- 

 quainted in the discussion of "false annual rings" and "moon rings." By 

 these terms are understood the brown circular, or zigzag stripes found by 

 testing microscopically the tissues injured by frost. These stripes are com- 

 posed of collapsed misshapen parenchymatous cells, and occur very often 

 but as yet have been but little studied. I have investigated more exactly 

 the phenomenon on branches of an apple tree which had been forced in a 

 greenhouse and then, in May, exposed for only 22 minutes to a temperature 

 of 4 degrees C. below zero. 



By the middle of June, in the experiments carried out on a branch of 

 which the tip was frozen, a sharp boundary was found between the dead 

 part and that which had remained alive. This observation is confirmed 

 in all frost injuries. A gradual extension of the injured zone does not 

 become noticeable subsequently, if no secondary factors, such as wood- 

 destroying fungi, enter into co-operation. However, the action of the frost 

 itself can radiate out into the healthy tissue in the death of certain parts as 

 was the case in the experiment under consideration. If the branch which 

 had died after its tip was frozen was cut off directly below the bud which, 

 adjoining the dead tissue, had remained healthy and liad sprouted, a 

 browned, sharply defined stripe was found to extend from the dead places 

 out into the healthy part of the axis past three healthy buds. This stripe 

 traversed the axis in a diagonal direction from the outside inward. 



The sharp limitation of the brown stripe and its diagonal course were 

 explained by a microscopic investigation. This proved that the main 

 vascular bundle of the lowest dead bud of the frozen tip is involved here. 

 This was, therefore, a case where the death of the bud gradually induced 



