40 CAUSE AND NATURE OF CROWN-GALL 



With the exception of less than a half dozen species, all slime- 

 molds described from North America belong to the subclass 

 Myxogastres, as defined by Macbride. 36 The many species of this 

 subclass are characterized by their abundant, minute unicellular 

 spores enclosed in more or less perfectly defined Sporangia. 

 With the single exception of the species described below, all 

 Myxogastres, so far as known, are saprophytic. 



Introductory to the description of the species believed to be 

 the cause of the crown-gall, the following may be given as de- 

 scriptive of the class as a whole : 



The Myxomycetes or slime-molds are common in moist situ- 

 ations in all parts of the world. They live, with rare excep- 

 tions, on decayed vegetable matter, and, as a rule, are extremely 

 minute. In the course of their life history they present two 

 phases, namely, the vegetative and the reproductive. The 

 vegetative phase is remarkable for its extreme simplicity of 

 structure, being nothing more than a nucleated mass of mov- 

 ing, naked protoplasm carrying with it occlusions of foreign 

 matter. It is amoeboid in its movements, and is known as the 

 ' ' plasmodial ' ' form of the organism. Under certain conditions 

 the plasmodia pass into transitory resting stages, to again come 

 forth when the conditions are favorable. After a variable 

 length of time fructification takes place, when myriads of ex- 

 tremely minute spores are formed, usually in a well-defined 

 spore-case or sporangium. The spores germinate shortly after 

 ripening, at which time the spore wall is ruptured, and a bit of 

 naked protoplasm comes forth as a zoospore or swarm cell, in- 

 distinguishable from an amoeba. After a few days the product 

 of the spores flow or creep together, and ultimately fuse to 

 form a new plasmodium, and the cycle of existence begins 

 anew. 



THE PLASMODIUM AND ITS TRANSITORY RESTING STAGES. 



In its plasmodial or feeding stage the parasite infests the 

 hypertrophied tissue of the gall and consumes the living proto- 

 plasm contained therein. The irritation caused by its presence 

 stimulates growth and accelerates cell division, resulting in a 



jC North American Slime-molds, p. 20. 



