﻿236 Grossenbacher: Medullary spots and their cause 



terminal one undergoes lateral enlargement and proliferates into 

 the available space, cutting off the outgrowth by a septum. An 

 early stage of this process may be noted near the left of Fig. 9. 



During the development of the substitute cambium over the 

 streak-wound the growth of the phloem cells usually reduces the 

 radial diameter of the mines from forty to sixty per cent. In the 

 case shown in Fig. 12'this compression growth was so pronounced as 

 to wholly prevent the usual form of occlusion. The phloem-ray 

 cells appear to retard this general growth that brings about such a 

 marked reduction of the channels, as may be seen from Figs. 6 

 and 7 which show the status of things in an early stage. The 

 broader rays have a more decided influence than the mono- or 

 biseriate rays. As a matter of fact, the same characteristics may 

 be noted in all the figures to a varying degree. The bark side of an 

 occluded mine nearly always has a marked undulatory boundary 

 line owing to the more active growth of the interray portions of 

 the bounding phloem that occurs during the development of the 

 substitute cambium over the wound. In some instances where 

 channels were made through a region in which the rays are all 

 monoseriate and at a time when radial growth is especially pro- 

 nounced, they may be entirely closed by general growth from the 

 bark and radial sides, thus leaving only the irregular lines of com- 

 pressed waste and some irregularities in the arrangement of the 

 cells to show the location of the medullary spot. That is, the 

 growth of the interray portions dominates the healing process and 

 thereby prevents proliferation from the rays, as shown in Fig. 12. 

 The boundary line along the pith side of the occluded mine is 

 usually fairly straight. This is especially noticeable in FiGS. 13 

 and 14. The tangential diameter of the occluded mine is from 

 one to four or more times the diameter of the miner. This greater 

 or tangential diameter of the original channel is also considerably 

 reduced during the early stages of the healing process, as may be 

 seen by the incurving of the bounding ray-cells, as well as by their 

 bulging enlargements encroaching on the mined side. Figs. 11, 

 15, and 16 show results under consideration. However, it is very 

 unusual for the uninjured ray-cells on the sides to give rise to pro- 

 liferations into the channels: the neighboring cells of the inter- 

 rupted rays seem to respond more quickly, apparently owing to the 



