28 MASS. EXPERIMENT STATION BULLETIN 183. 



in the collenchyma, the heavy walls of which seem to offer no resistance 

 whatever to the progress of the invader. Occasionally it has been found 

 even in the epidermal cells. The first bar to its inward progress is the 

 area of bast fibers. It does not pass through these at once, but in very 

 old cankers it has been observed even in the bast fibers. There is, how- 

 ever, an easy path between the bast areas through the flaring outer ends 

 of the medullary rays, which do not stop at 

 the cambium but extend up between the 

 phloem areas. From here the hyphce can 

 easily pass lateralh^ into the phloem. Pass- 

 ing down into the xylem elements the invader 

 finds its progress made much easier by the 

 jpr^^^a==f=^rjr-ji-jrJ^^^ prcseuce of pits in all the walls. It does not 

 ^'^^~^~^^>tf^^ confine itself to the medullary rays, but passes 

 Fig. 11. — Mycelium in the laterally into the other elements. The my- 

 ceiis of the medullary rays. celium has been fouiid in every element of 

 the X3dem, least of all, however, in the wood 

 fibers. Often in old cankers the tracheae may be found almost clogged 

 with mycelium, frequently in the form of chlamj^dospores. The method 

 by which it passes through the walls is shown in Fig. 11. From the 

 xylem it passes down into the pith, where it finds progress easj^ through 

 the thin walls. 



Effect on the Host Cells. — All of the cankers do not extend to the pith. 

 A great many of them, for some unexplained reason, never go deeper than 

 the bark. The fact that the affected plants stop growing, and do not 

 send up any more shoots from below the cankers, is probably due to 

 destruction of the phloem, which prevents any food passing down to 

 the lower stem or roots. The cells somewhat in advance of the invading 

 hyphoe first become filled with a brown, finely granular substance which 

 gradually becomes coarser and later mostly disappears, possibly being 

 used by the parasite, and the cells are left almost empty. The starch, 

 nuclei and chloroplasts also disappear. The walls are not affected except 

 for the holes through which the hyphce pass. The whole effect on the 

 host seems to be entire disorganization of the cell contents. There is no 

 hyperplasia, hypertrophy or other abnormal cell change in the canker. 

 To be sure, there is often a swelling just above the canker, which is pro- 

 duced by an increase both in the size and number of cells of the inner 

 cortex. This is, however, probably due to the amount of elaborated food 

 which is stopped here because it cannot now continue downward on its 

 normal course. As the canker becomes older, the cells of the bark col- 

 lapse, being now empty. The cracks which then appear in the bark may 

 be due to the contraction of the dying tissue, or to the expansion of the 

 growing stem, or both. The cells of the xylem and pith do not collapse, 

 but the affected tissues turn brown. 



