78 PROTECTION OF PLANTS, 1921-22 



Life History 



Spongospora suhterronea requires an abundant supply of moisture for germin- 

 ation, as well as warmth. Hence it is that the disease is more serious in poorly 

 drained soils or during highly humid seasons. The spores in the spore baU 

 germinate giving rise eventual!}' to naked masses of protoplasm known as "plas- 

 modia". A plasmodium moves slowly over a surface which is moist by means of 

 pseudopodia which are merely protrusions of the naked protoplasm. In this 



way, by moving in soil water among the 

 interstices of the soil, the plasmodia 

 reach roots, stolons and tubers. Which- 

 ever the part attacked, the plasmodium 

 penetrates the epidermis and gradually 

 passes down to the cells below. These 

 are stimulated to excessive growth so 

 that ''giant cells" are formed which may 

 also divide, thus giving an abnormal 

 number of cells in the tissue and ac- 

 counting for the gall formation on root 

 and stolon and the protuberance on the 

 tuber. In root and stolon galls the para- 

 site eventually reaches the phloem in 

 which most of the food manufactured is 

 conducted and in this tissue most of the 

 hypertrophy occurs. The tuber is some- 

 what modified because of its function as a 

 large storage organ and the plasmodia do 

 not, as a rule, reach the phloem but attack 

 cells of the hypodermis and cortex. 



Where a cell is covered by a plasmodium 

 or part of one, the wall swells and softens. 

 That it is changed chemically :s shown by 

 the modified staining. Through the soft- 

 ened wall the Plasmodium penetrates by 

 protoplasmic protrusions which Kunkel 

 calls "infecting pseudopodia". Once inside 

 the cell, the naked protoplasm of the myxomycete penetrates the limiting mem- 

 brane of the host protoplast and the two protoplasms are to an extent miscible. 

 It is when the cell is thus infected that hypertrophy begins. As the vegetative 

 growth of the potato gradually ceases and maturation commences the spores 

 of Spongospora subterranea are formed and these spores are aggregated into 

 round masses known as spore balls. Such groups of spores are highly resistant 



Fig. 5 — Roots of potato with galls caused 

 by Spongospora subterrar.ea 



