Dec. 23, i9i8 Parasitism of Cronartium rihicola 627 



of the tracheids ^ for a short distance (PI. 49, C). Such haustoria are 

 naturally limited in development. 



Radial longitudinal sections are more easily cut from bark or wood 

 than transverse or tangential longitudinal sections because radial cuts 

 are splitting cuts. Moreover, in radial sections the vertical widths of the 

 rays are exposed, and therefore all the infected cells of the rays, with the 

 haustoria they contain, as well as the adjacent infected phloem and 

 xylem cells, are more readily examined (PI. 53). The hyphae lying along 

 one phloem ray frequently are united with similar hyphae lying along 

 adjacent rays by connecting strands. The latter may work their way 

 between phloem parenchyma or sieve tubes. In the xylem the ray 

 hyphae may be connected in the same way by hyphae, which pass from 

 one ray to the other in the intercellular spaces between the tracheids. 

 Often the edge of the ray is bordered by a hypha lying in the space 

 between the outer ends of the marginal cells of the ray and the tracheids 

 (PI. 53). In the resin-duct parenchyma and the cells lining the duct all 

 the cells are usually penetrated by haustoria (PI. 53). This applies to 

 vertical ducts in the wood and bark and to horizontal ducts in fusion with 

 rays. 



The general features of the mycelium such as their more or less uniform 

 diameter, with occasional bulges where there is a little extra room in an 

 intercellular space, their relatively large size, uninucleate cells, and con- 

 stant relations to the host cells of the different tissues, are characteristic 

 enough to make the mycelium alone a sufficient basis for the recognition 

 of Cronartium rihicola before any spores are produced. The haustoria 

 often the most striking objects in the infected cells, are the most im- 

 portant elements of the mycelium from a diagnostic standpoint. The 

 haustoria apparently have the power to pierce the cell wall at any point 

 (PI. 53). Young haustoria are usually straight (PI. 58, B), constricted 

 at the point of passage through the wall, and irregularly bulging inside the 

 cell (PI. 53 ; 58, C, D). Their outline soon becomes more or less irregular. 

 In the phloem parenchyma they do not reach the development found in 

 the ray cells, where they coil on a wide spiral or curve at rather sharp 

 angles. The curve and the spiral are probably different expressions of 

 the same process — that is, the adjustment between the growing haus- 

 torium and the cytoplasm of the host cell. As Sappin-Trouffy (50, 51) 

 has pointed out, the haustoria of the rusts appear to seek out the nucleus 

 of the host cell, and sometimes even entwine it. No such extended 

 development has been observed in the case of the haustoria under dis- 

 cussion, but it frequently happens that the host nucleus is dented (PI. 53) 

 by the tip of the haustorium,. Olive {42) shows that the haustoria of 

 Botryorhiza hippocraieae Whetzel and Olive form botryose masses which 

 may almost completely fill the host cell. In this fungus the haustoria 



* In a previous article (7) the statement was made that haustoria do not enter the wood cells. Improved 

 technic has shown this statement to be an error. 



