GALLS AND TUMORS 



971 



of their chromatin and appear pale and indistinct, while attraction 

 spheres appear at opposite poles. The nuclei then divide twice by 

 karyokinesis and a small amount of cytoplasm is separated off, con- 

 stituting a gamete. The gametes now unite in pairs and each pair 

 becomes encysted to form a spore. 



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FIG. 200. Plasmodiophora brassica. A, A plant cell filled with parasites the 

 nuclei of which are undergoing mitotic division (at the top is the nucleus of the 

 plant cell). B, Two plant cells with developed and partly developed spores. (After 

 Prowazek, from Dofiein.} 



Whether during the multiplication of these organisms in the plant, 

 they are able to migrate to other cells and thus spread the infection has 

 been questioned. A number of investigators believe that the number 

 of infested cells is only increased by the division of the infected plant 

 cells which not only are greatly enlarged but also show evidence of 



