Myceliiiui of the Urcdinecs. 



action of caustic potash (KHO). The tubes themselves 

 are rather irregular in their outline, and branch at frequent 

 intervals. These branches unite with other mycelial tubes, 

 so as to form an anastomosing irregular network, which 

 perva:des more or less widely the tissues of the affected 

 plant. At rare intervals transverse septa are seen. The 

 function of this mycelial network is to utilize the elabo- 

 rated material which the host has prepared for its own use, 

 and to turn it into a suitable pabulum for the sustenance of 

 the fungus. 



The extent to which the mycelium permeates the 

 tissues of the host-plant varies ; in the majority of cases 

 it is localized and confined to a limited area. The germ- 

 tube from a single spore having once entered the tissues 

 of a leaf, the tendency of the mycelium thereby pro- 

 duced is to spread equally in all directions in a centrifugal 

 manner. Many causes, however, come into operation 

 which tend to prevent the ultimate spore products being 

 equidistant from the centre. This may be due, in part, to 

 the mycelial hyphae growing more luxuriantly in one 

 direction than in another ; but to a great extent it arises 

 from a want of uniformity in the tissues of the host-plant. 

 Still, however, we are often able to observe that the 

 fructification of those Uredines which have a limited 

 mycelial growth, is arranged either in a circle or in more 

 or less circular manner i^Puccinia lychnideartim, Uroinyccs 

 scillariim, Ccsoma orchidis, ^cidium zonale). In those 

 cases in which the mycelium is developed in a leaf with 

 strongly marked venation, this tends to exert a directive 

 influence upon its extension ; for example, the primary 

 uredospores of Triphragnimm idmari(£ and Uromyces 

 alcheitnllcB. The same directive influence of the tissues of 

 the host-plant is seen in the linear arrangement of the 

 sori of Pucci)na magnnsiaua, graiuinis, riibigo-vera, etc., 



