63 
nification each of the colourless swimming spores is’ seen 
iobe furnished with two excessively fine hair-like threads 
of protoplasm which, by lashing the water incessantly, 
bring about the movement of the spore. The active move- 
ments continue for twenty minutes or half an hour, then 
the spore comes to rest, rounds off and withdraws the 
whips of protoplasm. If favourably situated on a seed- 
ling it sends out a fine filament which bores its way 
through the outer wall of one of the cells and grows into 
the interior. It has been proved that the tip of the fila- 
ment is able to do this, because it secretes a substance 
which enables it to digest its way through cell walls 
pretty much as gastric juice renders our food materials 
soluble. Once within the cells of the victim the fungus 
branches and grows rapidly from cell to cell, spreading 
destruction as it goes and deriving nourishment from .the 
product of this destruction. In a diseased seed-bed these 
processes are going on so continuously that the fungus 
soon produces many thousands of the motile spores which 
are able to attack new seedlings, thus accelerating the 
progress of the disease. In addition, as we have already 
seen, even the threads of the fungus grow across the soil 
between the seedlings and directly produce new infection. 
It is well ‘known that “Damping off” will recur with 
even greater’ virulence in seed-beds which showed the 
disease the previous season. This is explained by the 
existence of yet another chapter in the life history of 
Pythium, by which resting spores are formed that can 
pass the winter in the soil. At a late stage in the decay 
of diseased seedlings many threads of the fungus give 
rise to the resting spores. These possess the power of 
lying dormant over a long period, and in this resemble 
the seeds of higher plants, though they only consist of a 
single thick-walled cell. When, however, the temperature 
is favourable and the conditions are sufficiently moist, the 
thick coat bursts and a fungal filament grows out which 
soon attacks any seedling that may be near, producing 
spore-cases and motile spores as before. The resting 
spores are produced in myriads in a diseased seed-bed ; 
in fact, on one occasion, I estimated the prcsence of upwards 
of half a million in a single diseased seedling observed 
under the microscope. As the seedlings rot these spores 
all find their way into the soil where they spend the 
winter. 
