STUDIES ON CLUBROOT OF CRUCIFEROUS PLANTS 433 



not as a plasmodium. There are several facts that prove this conclu- 

 sively, even tho the actual phase of the organism passing thru the wall 

 was never observed with certainty. A number of slides show cases that 

 might be interpreted as actual penetration, but as the nucleus in 

 no case appears in the act of making the passage one cannot be 

 certain of such an interpretation. Nevertheless, numerous cases are 

 to be found of a uninucleate amoeba just within the wall of the root 

 hair and far enough away from any other infection to preclude all 

 possibility of its having reached there except by entering singly thru the 

 wall (fig. 101). 



Evidently the reason why no one has recorded this stage heretofore is 

 because the amoeba hardly enters before nuclear division and growth 

 takes place. Some slides show binucleate amoebae still within the hollow 

 of the enlarged cavity, apparently produced by the stimulus of the para- 

 site. Other sections show trinucleate amoebae, and it is not difficult to 

 find amoebae with six or more nuclei (fig. 104, page 436). 



This series of stages would indicate that penetration takes place in 

 the uninucleate stage, particularly since the large multinucleate amoebae 

 are to be found, in nearly every instance, near the base of the root hair, 

 while the smaller and fewer-nucleate amoebae are always on the inside of 

 the root-hair wall about two-thirds of the distance from the base. Amoebae 

 are seldom found in the tip of the hair. 



Another point that confirms the above view of penetration is that in 

 the absence of growing host roots the swarm-spores develop no further 

 when the spores are germinated under artificial conditions, and after a 

 short period of activity the swarm-spores encyst and eventually die. 

 If plasmodia are formed under normal conditions, there should have been 

 at least a suggestion of this in a few of the numerous cultures used in 

 the experiments. 



In this connection also the very interesting question of sexual fusion 

 arises. It is believed by several cytologists that there are two nuclear 

 divisions just before spore formation and that one of these is probably a 

 reduction division. If this is true, it would imply that somewhere in the 

 life cycle there has been a fusion. Winge (1913) and others believe that 

 this occurs among the swarm-spores before they enter the host. Prowazek 

 (1905) is of the opinion that the amoebae within the host unite and then 

 the nuclei fuse. Even Nawaschin (1899) believes this union takes place, 

 but apparently he thinks it is of no significance in reproduction. Maire 

 and Tison (1909, 1911) have disproved the amoebal union, and their view 

 is certainly correct, for it is possible to find slides showing one amoeba 

 breaking up into spores while in another, immediately adjoining, division 



