June 15, I9I5 Spongospora Subterranea 267 



not sharply limited, but gradually faded out along its edges. On the whole, 

 these spots, each surrounded by a faintly grayish colored, translucent 

 ring, presented much the appearance that might be obtained by inject- 

 ing a small drop of water beneath the epidermis of a young tuber. A 

 study of free-hand sections through some of these spots led to the conclu- 

 sion that they were caused by a small, disk-shaped drop of light grayish 

 colored material just beneath the epidermis. Some of these spots were 

 fixed in Flemming's stronger solution and were labeled "halo stage of 

 Spongospora," because when held in the sunlight the small brownish spot 

 had the appearance of being surrounded by a halo. Some of this material 

 was sectioned and stained along with other early stages of the disease 

 before plasmodia had been observed in culture. Saprophytic plasmodia 

 produced in culture media have been studied parallel with investigations 

 of the microtome sections through the translucent areas. 



Early and late stages of infection have been obtained, and many of 

 the slides show with great clearness that the potato tissue is first invaded 

 by the fungus in its plasmodium stage. The light-brown spot at the 

 center of the translucent area above described is undoubtedly the point 

 at which the plasmodium enters the skin of the tuber. The limits of the 

 translucent area mark the distance to which the plasmodium has spread 

 beneath the epidermis. 



In stained sections it is possible to observe in detail the manner in 

 which the potato tissue is attacked by the plasmodium. Some of the 

 preparations show it passing down through and between the epidermal 

 cells. Usually a considerable number of cells are killed at the point 

 where the plasmodium enters. Once beneath the epidermis, it spreads 

 out in all directions and forms a rather flat, disk-shaped mass, which 

 separates the epidermis from the tissue beneath. In general, the potato 

 cells in contact with the plasmodium are at once stimulated to abnormal 

 growth and division, but some of them are killed as the plasmodium 

 spreads out over the healthy tissue. In this w^ay it comes to occupy a 

 space between the uplifted epidermis and sound tissue beneath. This 

 circular plasmodial mass is thicker at the point where it enters the epi- 

 dermis than toward the edges. At first the lower surface of the disk- 

 shaped mass is almost smooth. It comes in intimate contact with the 

 cells on which it rests. Soon, however, a number of projections of 

 pseudopodia begin to extend downward, push in between the cells of 

 the sound tissue, and seem to crowd them apart. All of the cell walls 

 that have been or are in contact with the plasmodium stain differently 

 from those in healthy tissue, and such cell walls are often somewhat 

 swollen, showing a special affinity for the orange stain of the triple com- 

 bination. This indicates that the cellulose is being acted on and in 

 some manner changed, but whether the change is accomplished through 

 the secretion of an enzym the writer is unable to say. It is possible that 

 91007°— 15 6 



