MILDEWS, RUSTS AND SMUTS 
53 
A mushroom, as generally understood, no more represents 
the entire fungus than an apple represents the entire apple 
tree. The mushroom is only the fruit of the fungus, as the 
apple is the fruit of the apple tree. It is known that when 
a mushroom-bed is prepared, the material called spawn 
has to be introduced before a crop of mushrooms can be 
produced. Now spawn consists of a dense mass of very 
fine, colourless threads, called hyphae (sing. Jiypha). These 
hyphae collectively constitute the mycelium and the my¬ 
celium in turn constitutes the vegetative portion of the 
fungus. Its function is to provide food for the growing 
plant, and is usually imbedded and hidden in the matrix 
or substance on which the fungus is growing, hence it is 
not generally observed, and the spore-bearing portion, 
which is produced on the surface, is considered as the entire 
fungus. 
When a spore of one of the uredines alights on a suitable 
leaf under suitable conditions, it usually germinates within 
twenty-four hours, and emits a slender hypha, called the 
germ-tube ; after increasing in length for some time the tip 
of the germ-tube swells into a vesicle just over a stoma 
of the leaf. This vesicle is called the appressorium, into 
which the nucleus and contents of the spore pass. A slender 
tube from the appressorium then grows through the open¬ 
ing of the stoma into the air cavity, situated just underneath 
the stoma. In this cavity the slender tube passing through 
the stoma forms a second vesicle into which the contents 
of the first formed vesicle pass in turn, and from this 
vessicle originate the hyphae which spread in all directions 
between the cells of the leaf. The hyphae running between 
the cells of the leaf, give off at intervals short lateral branches, 
which pass through the walls of the cells of the leaf, for the 
purpose of absorbing food from the contents of the cells. 
These branches are called haustoria (sing, haustorium). 
Haustoria are of various forms. In some species they do 
not differ in form from ordinary hyphae, in others they are 
more or less lobed, or swollen at the tip. These details 
are sometimes of generic, or specific value. Haustoria are 
frequently difficult to see in a section, but if a dilute solu¬ 
tion of eosin is run under the coverslip, the haustoria be¬ 
come stained a beautiful ruby-red colour. 
Reproductive organs. 
Nowhere else in the Vegetable Kingdom are so many 
