6l2 



The browning does not extend into the surrounding tissue, as in wound 

 rot, for the cells already existant, as well as those formed later in the imme- 

 diate proximity of the tissue browned by frost, remain clear-walled and 

 healthy. The browned medullary cells contain as much starch as do those 

 not attacked, so that the brown color can not arise from a change in the 

 starch but from some other substance. The pith does not suffer in every 

 case. Often the wood in 2 to 3 year old branches is so browned that a 

 yellow, gum-Hke filling of the ducts extends up to the medullary crown and 

 the medullary rays also appear brown almost to the centre; the pith itself, 

 however, having no diseased discoloration whatever. Such differences 

 take place in different internodes of the same branch. Nevertheless, the 

 rule holds that the initial stages of browning are found, on an average, in 

 scattered cells of the pith, especially those of the pith crown ; that, at first, 

 only the contents and then later the walls themselves become discolored and 

 that this discoloration of the contents seems to consist of a browning and 

 thickening of the cell fluid. The gum-like solid masses can break in sec- 

 tioning into angular pieces. I believe the filling of the ducts must be traced 

 back in part to the hardening of the fluid contents already existing, in this 

 way easily explaining the often drop-like formation of the filling substance. 



With increasing cold the browning of the pith, as a rule, follows that 

 of the medullary rays and bast parenchyma groups in the bark. In branches 

 of the red beech frost action, limited to individual vascular bundles, can 

 often be recognized; the discoloration is restricted to the inner half of two 

 main medullary rays, attacking first the part of the bundle which belongs 

 to the medullary crown and often ending suddenly with the boundary of an 

 annual ring. 



At times the wall of the duct may be found unstained or only discol- 

 ored on one side, while the contents seem completely discolored. It was 

 mentioned above that the secondary membrane can also participate in the 

 filling of the ducts and wood cells. At first this swells up and at times, in 

 fact, completely fills the lumen of the wood cell, or of the narrow duct, 

 which then seems colorless and refracts the light uniformly. Besides this, 

 cells and ducts are found which have turned a deep brown ; their cell con- 

 tents often lie in the form of drops or rings against the wall, but sharply 

 separated from it. In other cases there is no separation between the cell 

 contents and the cell wall and here the participation of the wall in the 

 change is certain. It also may happen that only an inner layer of the cell 

 wall turns brown, swells up and finally becomes rigid. This swollen layer 

 then has not space enough on the inner side of the cell, or of the duct, and 

 folds inward so that a colorless cavity is found between the brown wall 

 layer, which has been pushed inward, and the outermost, unchanged portion. 



In the browning of the cambium, which usually occurs only on one side, 

 the contents are only slightly browned and the cell wall does not discolor at 

 all until later. The spring wood, directly adjoining the autumn wood, seems 

 to be most sensitive. It is evident in the bark, that the parenchymatous 



