382 Cannon : Anatomy of Phoradendron villosum Nutt. 



the host and their later development into sinkers, it will be seen 

 that the haustoria seek only those cells which contain food ; they 

 are chemotactic, and this may be called the positive element in the 

 erowth of the haustoria. The negative element is found in those 

 host cells which are either poor in food, or furnish mechanical re- 

 sistance to the progress of the haustoria, or both. The cells to- 

 ward the periphery of the branch would represent the first, and the 

 grit cells and fibers the second conditions. How masses of grit 

 cells may affect the growth of the haustoria has already been 

 touched upon. It was found, however, that owing to the character- 

 istic form of these cells the haustoria could envelop them with their 

 own tissues and flow past them, the course of growth not being 

 changed by their presence. The case is somewhat different in re- 

 gard to the fibers in the cortex of the oak, and they are an im- 

 portant factor in directing the growth of the cortical haustoria as 

 well as those haustoria which penetrate into the xylem of the host. 

 The position and number of sinkers are to a great degree, and 

 perhaps wholly, controlled by the permeability of the inner ring 

 of fibers to the haustoria. From the outer cortical cells of the 

 host the haustoria grope between masses of fibers which compose 

 the outer ring of sclerenchyma to the parenchyma within (Fig. 13)* 

 and in a similar manner through the second and each ring of fibers 

 and parenchyma in succession until the inner ring is reached, and 

 the haustoria pass between the gaps in this ring and penetrate the 

 bast and become transformed into sinkers. In every instance the 

 haustoria pass from an area poorer in food to one richer, and they 

 are led in this manner to a position opposite the medullary rays so 



a 



that with an increase in diameter of the host stem they are in posi- 

 tion to supplant them. Such in brief is the origin of the sinkers. 

 A young sinker is in no respect different from any other part of 

 a cortical haustorium. It originates on that part toward the wood 

 cylinder and, as has just been stated, becomes a sinker by penetrat- 

 ing between some gap in the inner fiber ring of the host cortex. 

 When the phloem of the host is reached, and especially the cambium, 

 the cell walls of the sinker become somewhat heavier and the end 

 cells shorter than corresponding ones of the cortical haustoria (Fig. 

 10). The host cells just in front of the tip of the sinker in the cam- 

 bium or in the bast present a curved appearance as if under pres- 



