IOO 



FUNGOUS DISEASES OF PLANTS 



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 V.: !' 



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Morphology. Fungus and deformity. The parasite is sup- 

 posed to gain entrance to the host plant during the swarmspore 

 stage, or immediately upon leaving the swarmspore stage, there- 

 fore in the amceboidal form. No observations, however, have been 



made relative to 

 host penetration, 

 and the subject 

 would doubtless 

 prove an interest- 

 ing one. 



A microscopic 

 study of sections 

 of the diseased 

 root shows that 

 the organism is 

 most abundant in 

 ^W^X ^ parenchymatous 



i j $S . cells > often in the 



vicinity of the 

 cambium. There 



FIG. 21. STAGES IN THE DIFFERENTIATION OF THE is in quantity an 

 PLASMODIUM AND SPORES IN PLASMODIOPHORA BRASSIC.-E abnormal develop- 

 (After Nawaschin) 



ment of phloem. 



The xylem portions of affected roots are relatively inconspicuous. 

 According to some observers, certain bundle elements may also 

 show the parasite. 



The infested cells are ordinarily in groups (Fig. 20) and 

 Nawaschin believes that these groups originate by the division 

 of a single cell and that such groups may also transmit an in- 

 fluence to similar tissues even at a distance, so that there may 

 eventually result, for instance, histological disturbances in neigh- 

 boring bundles. It is possible, however, that the young cells of 

 the bundles may become infected and that the organism may be 

 enabled to maintain itself in such cells for a time after differentia- 

 tion of the latter as distinctive bundle elements. 



In an earlier stage the contents of the infected cells is of a half- 

 fluid consistency, later turbid, and finally granular. Even in the 

 first stage the parasite is noticeable in the amceboidal form and 



