164 
These amebae soon fuse to form a small plasmodium which penetrates 
through and between the epidermal cells of the tuber. If conditions are 
not favorable to the formation of a plasmodium, the individual amebae 
may encyst. Later, when conditions become favorable, each protoplast 
escapes from its cyst-wall through a hole and may fuse with others to 
form the plasmodium. 
Once within the tissues, the plasmodium spreads out under the epidermal 
tissues and down between the cells of the cortex. Study the prepared 
slides provided; or Jour. Agr. Research 4, pl. XX XIX, fig. 3-5, and pl. 
XL. OBSERVE :— 
16. The densely granular protoplasm of the plasmodium in the 
intercellular spaces just beneath the epidermal layers. 
17. The pseudopodia-like extensions of the plasmodium, forcing 
their way down between the cortical cells. The middle-lamella is dis- 
solved and the cells are forced apart. The walls become soft and swollen. 
18. That the pseudopodia penetrate the cell-walls and portions 
of the plasmodium enter; these become separated from the main body of 
the plasmodium. 
Make a pRawinc to show the invading plasmodium, outlining the 
adjacent host-cells. 
‘After a certain period of feeding, all the plasmodia in the host-cells 
of a particular lesion simultaneously form spore-balls. Examine prepared 
sections through sori showing mature spore-balls; or study U. S. Agr. 
Dept. Bul. 82, pl. I. opservE-— 
19. That the entire plasmodium. is converted into spores. 
The host-cell serves as a sporangium. 
20. The average number of spore-balls in a host-cell. 
21. Remnants of the host-protoplast; nucleus, starch-grains, 
or cytoplasm. 
22. Host-cells in which spore-formation is incomplete. Com- 
pare nuclei and cytoplasm of the pathogene and host. 
Make enlarged and detailed prawines of the host-cells, (a) containing 
plasmodia; and (b) containing mature spore-balls. 
Saprogenesis. What takes place after the spore-balls are set 
free in the soil is not certainly known. They probably germinate very 
shortly, the amebae passing out into the soil where they either form a 
plasmodium or encyst separately. Whether the organism can live a 
true saprophytic life in the soil is not known. It has, however, been 
cultured on artificial media. 
Secondary Cycles probably occur, as the spores can apparently 
germinate as soon as they are mature. The phenomena of the secondary 
cycles are probably the same as those of the primary cycles. A peculiar 
type of secondary infection of the deeper tissues of the tuber, surrounding 
a mature sorus, has recently been described as the cause of the canker- 
form of the disease. (See Kunkel, Jour. Agr. Research 4:272-273.) 
Pathological Histology. There is a marked stimulation of the host- 
tissues by Spongospora subterranea, as may be seen by the comparative 
study of the cells in the diseased and healthy regions of the tuber. 
Cut thin cross-sections (or use prepared slides) through the healthy 
potato skin. Examine and opsERvE:— 
23. The single layer of thin cells on the outside,—the epidermis, 
overlaid by the brown cuticle. 
