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bracket fungi and the honey agaric, for instance, live as 
parasites on trees. The vegetative filaments of these 
ungi ramify in the tissues of the tree, gaining entrance 
often through wounds, and only the reproductive bodies 
are produced outside. But very many diseases of plants 
are caused by fungi which, like the saprophytic mould 
Mucor, are so small that they can only be studied with 
the aid of the microscope. Such a minute parasitic fungus 
called Pythium de Baryanum often causes the “ Damping- 
off ” of seedlings. 
The “Damping-off” disease very frequently appears 
when the seeds of many plants are sown too thickly and 
grown under conditions which are too warm and moist. 
Young seedlings begin to die off in patches and soon 
ptesent a very characteristic rotten appearance. The disease 
1s very commonly met with in gardens and greenhouses, 
occurring in seed-beds of all kinds a few days after the 
germination of the seeds. It is most abundant in very 
wet weather and when the beds are kept too shaded and 
-badly ventilated; crowding of seedlings also favours the 
rogress of the disease. At first a few individual seed- 
ings are attacked at or near the surface of the ground, 
the tissues in this region having a water-soaked appear- 
ance. Soon the cells collapse, and being unable to 
support the weight of the cotyledons the seedlings 
fall prostrate. Those immediately around are similarly 
affected, the disease spreading through the seed-bed in 
ever widening circles until practically the whole may be 
destroyed. When the plants fall over they become pale 
and rotten, and soon the whole bed is reduced to a moist 
mass of decaying seedlings. This mass is seen, on closer 
examination, to be covered with the very delicate threads 
of a fungus somewhat like the mould on bread. Often 
the filaments can be seen to have spread from the first 
diseased seedlings to the outer parts of the circle. These 
filaments most often belong to the fungus Pythium, which 
is so called because of its ability to produce rotting. The 
fungus continues to grow in the dying seedlings, and the 
filaments may form a dense felt over the whole seed-bed. 
If a seedling is examined under the microscope shortly 
after it is attacked, the collapse of the tissues just above 
the ground is seen to be due’ to the destruction of the 
cells at that spot. This destruction is caused by the fila- 
ments of the fungus which at this stage are seldom visible 
