DAMPING OFF. 237 



older portion, and has the advantage of requiring a much less ex- 

 penditure of energy than if the opening were made of the same 

 size as the hypha. After passing through the cell wall the hypha 

 enlarges to the normal size. 



While the mycelium is comparatively young the inner portion 

 of the hypha is continuous, i. e., there are no cross walls parti- 

 tioning the tubes into sections. This is a characteristic possessed 

 by a very large group of fungi to which the Artotrogus belongs, 

 known as the Phycomycetes. The protoplasm within the hypha is 

 finely granular when the mycelium is young, but in the larger 

 threads as they become older the granules become coarser, their 

 contents are not so homogeneous, and the granules tend to collect 

 into groups or very irregular masses, somewhat resembling the 

 protoplasm in some mucors. 



In a crowded seed bed after a few plants have fallen, unless the 

 disease is checked, it will spread from these affected ones as cen- 

 ters to others near them and thus from the one or several starting 

 points the plants will fall until nearly or quite all of them have 

 been killed. Where the soil and atmosphere is quite damp and 

 the temperature conditions so high as to favor rapid growth of 

 the fungus it will grow out from the diseased part of the stem into 

 or on the surface of the soil for a few millimeters in extent as a 

 very delicate cottony mass or velvety pile. Where the adjacent 

 plants are not too far distant the superficial threads may thus 

 reach them and communicate the disease to them. In other cases 

 minute motile reproductive bodies called zoospores, or swarm spores 

 (perhaps more properly zoogonidid] are developed in a manner to 

 be described later. These swim in the soil water to the more dis- 

 tant seedlings and thus spread the disease. 



Sometimes there will be seen quite a profuse growth of a my- 

 celium, which on the surface of the soil may spread several cen- 

 timeters in extent. Usually this profuse growth is that of another 

 fungus, a Rhizopus, or Mucor, or in other cases a different ' ' damp- 

 ing off" fungus to be described in a later paragraph. 



If the tissues examined as described above from a seedling 

 which has not remained long after falling over perhaps the con- 

 dition of the mycelium described will be the only phase of the 

 plant (for the fungus is a plant) at that time present. If it has 



