W. Robinson 129 



The Mycelium in the Tissues. 



The initial infection of aster plants by the Phytophthora occurs in 

 the seedling stage through the roots and root- hairs. This was proved 

 by observations on seedlings shortly after infection as well as by the 

 infection experiments which are described below. Many diseased 

 seedlings wilt and succumb almost immediately but others, though 

 infected by the fungus, continue to grow and may even reach the 

 flowering stage before wilting. A comparison of sections of the stem 

 of diseased asters of various ages shows that the wilting, both of seed- 

 lings and of older plants, results from the extension of the mycelium 

 to the tissues of the vascular cylinder. The presence of the fungus in 

 the older plants, even though the wilting is delayed, invariably produces 

 serious dwarfing. This effect is obvious in Plate XXV, which is a drawing 1 

 to scale from a photograph of two plants of the same age, the one 

 attacked by the fungus and the other free from it. Both plants were 

 grown side by side under similar conditions in the same bed. 



Transverse and longitudinal sections of diseased plants of different 

 ages show that the mycelium advances upwards in the cortex and for 

 a time at least the cells of these tissues remain living and turgid. The 

 mycelium grows both in the intercellular spaces and through the cells, 

 and in the latter case, when entering or leaving a host cell, a hypha 

 shows a distinct constriction where it passes through the cell wall 

 (Plate XXVI, figs. 1 and 2). Suitably stained preparations show that 

 the hyphae penetrate through the small pits which are frequent in the 

 cell walls (Fig. 3). Haustoria rarely occur, if indeed they are present 

 at all. This is difficult to decide with certainty since branches of the 

 hyphae grow through the cells and the apparent haustoria may simply 

 represent such young branches. Sooner or later the mycelium extends 

 to the vascular bundles. The thin- walled tissues of the phloem especially 

 are attacked and rapidly killed; the hyphae penetrate into the cells 

 of the medullary rays (Fig. 4) and even occasionally send branches 

 through the vessels (Fig. 5). The passages of the last named, however, 

 are not directly clogged by the fungus as was suggested by R. E. Smith 2 ; 

 but the proximity of abundant mycelium to the vessels, as well as the 

 drain on the phloem and medullary rays, is sufficient to account for the 

 wilting. 



The mycelium consists of sparingly branched hyphae from 3/x to 



1 I am indebted for this drawing to my brother Mr J. B. Robinson. 



2 Bull. Mass. Agric. Coll., 1902. 



Ann. Biol, n 9 



