626 Journal of Agricultural Research voi. xv, no. 12 



in some cases seems to vary directly as the succulence of the host. It 

 is at this time impossible to determine what part weather conditions 

 play, although it seems evident that, once infection has taken place, 

 external conditions cause little or no change in the growth of mycelium. 

 The time of production of gecia in the spring varies directly with warm 

 and favorable weather conditions. 



MYCELIUM IN PINUS STROBUS 



A brief summary of the main characteristics 01 the mycelium and the 

 relation of the parasite to the cells of the cortex and phloem of the 

 pine host has already been published (7); however, both subjects will 

 bear review in greater detail. No data can be given at this time on the 

 maxmer in which the mycelium enters the host tissue, because the 

 youngest stages of infection have not been observed. The course of 

 the hyphae is always intercellular (PI. 53). Conditions found at the 

 edge of the infected area indicate that the fungus makes its advance 

 first in the most recent phloem parenchyma and in the rays. This 

 holds true for infections of several years' standing, as well as for those 

 which, to judge from the size of the diseased area, could not have been 

 more than a year old. The hypha are relatively large, 3 to 5.5 /^ in 

 diameter, and are divided by cross walls into cells which vary con- 

 siderably in length (PI. 53). Each cell has a single conspicuous nucleus. 

 In transverse and radial sections the hyphae can be traced along the rays 

 past the cambium into thexylem (PI. 53). Here they are confined to 

 the region of the ray cells, with the exceptions noted below. Because 

 of difficulties in cutting transverse sections, which are at right angles 

 not only to the "grain" of the phloem but also to the general course 

 of the hyphae, such sections are not favorable for study, except in com- 

 parison with other sections cut tangentially or radially. In tan- 

 gential longitudinal sections of the cortex and phloem the myceUum is 

 frequently found in strands, which are especially prominent in the outer 

 phloem region. The strands may surround groups of the phloem 

 parenchyma. In some cases the older sieve cells appear to be filled 

 with hyphae; but careful observation shows that they have been split 

 apart and flattened out by the hyphae, which have forced their way 

 into the enlarged intercellular space, and thus practically occupied 

 the same amount of space that was formerly occupied by the living 

 sieve cells. In the rays the hyphae frequently fill the intercellular spaces 

 adjacent to the horizontal walls of the ray cells, and haustoria from 

 these hyphae penetrate the ray cells. Resin cells are also penetrated 

 by haustoria. In the xylem the same general conditions are met with 

 in respect to the ray ceils that are found in the phloem. Short haus- 

 toria arising from the hyphae in the angles between these ray cells bore 

 through the thin walls of the adjacent tracheids and enter the lumens 



