Feb. 3, 1919 Physoderma Disease of Corn 147 



DEVELOPMENT OF THE FUNGUS WITHIN THE HOST TISSUE 

 So far as is known, the fungus is an obligate parasite. After zoospore 

 germination and host penetration the fine mycelial fibers invade a number 

 of the surrounding parenchyma cells, forming numerous enlarged cells, 

 or Sammelzellen (PI. 15). These enlarged cells are always intracellular, 

 and are often in groups of two or more. A number of the small fibers 

 are usually found extending in various directions from these cells. These 

 fibers penetrate the host cell walls at any point (PI. 16, a-e) , passing directly 

 into the adjoining cells which are likewise invaded through the same proc-' 

 ess of enlarged cell and fiber formation. Commonly where the host cell 

 walls are penetrated the cell wall and the hypha of the fungus seem to be 

 unmodified. In some cases, however, there may be a slight enlargement 

 of thejnyceUum or a slight thickening or modification of the host cell 

 wall, or perhaps both (PI. 16, a-e) . The hyphae of the fungus are so small 

 that no opening can be seen where they pass through the cell walls. 

 The enlarged cells of the fungus may apparently develop directly into 

 sporangia or send out short fibers which produce a single terminal 

 sporangium. In cases where the enlarged cell develops directly into 

 sporangia there is a rounding up of the content of the enlarged cells 

 around a denser part of the protoplasm, which is to all appearances a 

 nucleus. Where fibers grow out to produce terminal sporangia, they may 

 arise directly from the enlarged cells without any noticeable disturbance 

 to the nuclear structure. In some instances a double nucleate condition 

 is seen, and a thread or fiber develops from this structure to produce a 

 terminal sporangium (Pi. 16, ^). A very common form of the enlarged 

 body is an elongated structure containing from two to four cells (PI. 16, /,9), 

 but more commonly two cells. These structures, which have very thin 

 walls and less dense protoplasm than some of the more compact struc- 

 tures, are often found to produce sporangia on the thin fibers. Where 

 there are only two cells present one may give up its contents and collapse 

 while the other may develop directly into a sporangium or produce a 

 sporangium at the end of a hypha. In some cases both cells reproduce 

 in one way or the other. The same is true for the three-celled and four- 

 celled bodies, one or two cells of which apparently may collapse while 

 the others produce sporangia. The double nucleate condition was more 

 noticeable in structures of this kind. 



After the formation of sporangia is complete, the invaded host cells 

 are usually filled with them (PI. 1 7, B) . No traces of the mycelium or vege- 

 tative cells can be seen at this stage. These parts are entirely absorbed 

 or broken down in the process of reproduction. The host cells appear to 

 be slightly enlarged, owing to invasion by the fungus. They die as soon 

 as the sporangia are formed, as their protoplasm is almost completely 

 destroyed by this time. In the early stages of invasion there is a stimu- 

 lation of the invaded cells which is brought about by the presence of the 

 parasite. This no doubt accounts for the noticeably slight enlargement 

 of cells. 



