1902.] NATURAL SCIENCES OF PHILADELPHIA. 493 



branches, if early iufected, are more badly diseased, as evidenced by 

 the larger size and confluent condition of the flead tissues, than the 

 heavier branches, if infection takes place after the branch has reached 

 considerable size. The writer has no evidence that the disease 

 spreads down into the wood of the older portion of the branch 

 from the smaller branches by the longitudinal growth of the 

 hyphge, although it is within the range of probability that this 

 downward growth does take place. 



C0NCLUDIN(; ReMAPvKS. 



That the metabolic activities of the cells invaded by the mycelia 

 of ihe two parasites above described are changed from the normal 

 condition is proved by the accumulation of material in the trach- 

 eids and medvdlary ray cells influenced by the presence of the 

 fungi. The nature of these accumulated substances has already 

 been discussed. Not only are the metabolic activities of the host 

 cells altered, but the cambium in which the mycelia lives is stimu- 

 lated to increased divisional activity, and this stimulation may exert 

 itself to some distance. Townsend"* has shown that " the influence 

 of an irritation, due to cutting or other injury, is capable of acting 

 through a distance of several hundred millimeters." It would 

 seem, therefore, that plants that are victims to parasitic fungi may 

 possibly be influenced as if they were wounded. How this 

 increased activity of the host cells expresses itself in the increase 

 in the amount of wood and bast has already been discussed. It 

 appears that the fungi perennate in the wood of the canker, form- 

 ing there a loose open reticulum, much like a coarse fish-net, and 

 that they cause an alteration in the activities of the cells, obtaining 

 for themselves thereby a suflicient amount of food for continued 

 slow growth. The hyphae which are instrumental in the formation 

 of the swellings clearly reside in the wood cambium and adjacent 

 soft bast cells, being able to draw upon the supplies of that part of 

 the mycelium which has lived longest in the stem. It appears 

 then that the mycelium of the wood was once as active as the 

 mycelium of the cambium, and that as the permanent tracheids and 

 medullary ray cells were formed the ^^alls of the hyphte increased 



28T0WNSEND, The Correlation of Growth Under the Influence of 

 Injuries, Annals of Botany, XI, pp. 509-533 (18t)7). 



