STUDIES ON CLUBROOT OF CRUCIFEROUS PLANTS 425 



infection takes place at a temperature of from 16 to 21 C. The pres- 

 ence of the host seems in some manner to exert an influence which to a 

 certain extent takes the place of that offered by a greater amount of heat. 



Usually the first sign of germination is a swelling of the spore, which 

 sometimes becomes a third larger. This occurs within a period of from 

 fifteen minutes to eight hours after the spores are placed in the medium, 

 altho the best time for examining the culture proved to be at the end of 

 six hours. After the swelling of the spore there is a bulging at one side. 

 The protoplasm withdraws from near the opposite wall and leaves a 

 nearly hyaline semicircle about two-thirds of the distance from the center. 

 The pressure exerted splits the wall just enough to permit the protoplasm 

 to ooze out. Unlike Woronin (1878) and Mangin (1902), the writer has 

 never observed the protoplasm taking the various shapes that these 

 authors assign to it, but while oozing out it collects in a sphere or a hemi- 

 sphere against the wall on the outside. When about half of the proto- 

 plasm has escaped, the whole body becomes motile. At first there is only 

 a trembling, which gradually increases in violence until the spore is turned 

 around entirely. The activity now becomes so great that it is with diffi- 

 culty that the microscope is kept focused on it correctly. The final struggle 

 is apparently a rapid spurt across the field, when the swarm-spore is lib- 

 erated from its container, and at once begins its rotatory activities. The 

 whole process under the microscope consumes an hour or longer. Evi- 

 dently the strong light turned on a spore retards the action, for in many 

 cases the spores that had begun to germinate when placed in view showed 

 no further signs of development, while those kept in the dark germinated 

 much more rapidly and when examined at the end of the same period 

 were found actively swimming about. 



A considerable part of the contents is left within the old spore wall, 

 so that when the broken part is turned upward it has the appearance of 

 a circle bounded by a darker band, the width of which is about one-third 

 of the radius. If, however, the open part is on the side, the residue within 

 the spore wall resembles more nearly a crescent (fig. 97). 



The swarm-spore when alive measures from 1.7 to 3.5^ in length, 

 being more or less pyriform with a thick flagellum at the smaller, or 

 anterior, end and a vacuole near the posterior end. Unless stained, the 

 flagellum cannot be seen under the microscope. The line of locomotion 

 is never a straight one, for the flagellum is lashed about by the beak, 

 which is constantly doubling backward so that a whirling motion is given 

 to the swarm-spore. Altho the latter is a naked mass of protoplasm, 

 the writer has never seen the various shapes which Woronin (1878, PL 

 xxxiv) has pictured; it was observed in every case to be globose or pyri- 

 form, never having pseudopodia-like structures. 



