Plant Diseases 
325 
into the next cell by absorbing the cell wall. In this way 
it works right through the tissue of the small seedling. A 
network of hyphse is formed by this time. This is known 
as the mycelium or dis- 
ease plant. When the 
disease has made its 
way through, the seed- 
ing drops over, and 
soon dies. In the mean- 
time the mycelium con- 
tinues to grow, and 
comes again to the out- _ m 
side of the plant tissue to 
form fruiting branches. 
At the apex of each of 
these branches a little 
spore is formed, called 
a conidium. These 
conidia, on germinat- 
ing, produce a little 
hypha which, entering 
another seedling, again Damping-off Disease 
A, Seedlings with plenty of room. B, Seedlings 
overcrowded, and attacked with Pythium. C, Seedling 
attacked with Pythium at ground level. D, Part of 
tissue of stem, showing how Pythium destroys the 
plant. E, Part of hypha bearing conidia. F, Conidia. 
G, Zoospores being liberated. 
crives rise to the dis- 
o 
ease. They may, how- 
ever, on germination, 
produce only a small 
tube with a sac at the 
apex of it, the contents of which divide into a number 
of small spores, each with two cilia attached. These are 
able to swim about in any moisture which may be 
present, and for this reason are called zoospores. These 
