Mycelium of the Uredinee. 5 
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 
pervades 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 toa 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 hyphe 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 (Puceinia lychnidearum, Uroutyces 
scillarum, Caoma orchidis, Atcidium 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 Triphragmium ulmarie and Uromyces 
alchemille. The same directive influence of the tissues of 
the host-plant is seen in the linear arrangement of the 
sori of Puccinia magnusiana, graminis, rubigo-vera, etc., 
