BOT.— Vol. II.] PEIRCE— ROOT-TUBERCLES. 299 



It is well known that many bacteria are actively motile only 

 for a short time after division. When the conditions in the 

 soil — whatever these conditions may be — favor the repro- 

 duction of these bacilli, the young and separate rodlets may 

 be able to move with a fair degree of rapidity. They may 

 possibly be stimulated into motility by the proximity of the 

 root of some leguminous plant, and may be characteristically 

 attracted to it by the substances diffusing from it into the 

 soil-water (see Czapek, 1896). At first glance this seems 

 improbable because of the very small proportion of root- 

 hairs attacked to the total number of root-hairs formed; 

 but on this point the following observations throw some 

 light. 



In the field of a Leitz objective III and ocular 3 I counted 

 one hundred root-hairs on the sides of a young lateral root. 

 Of these hairs one was infected. There must have been an 

 equal number of root-hairs on the top and bottom of the 

 root as it lay on the slide. The zone of hairs was about 

 five times as long as the distance through which I counted 

 hairs. ^ This would make the total number of hairs on this 

 small root at least one thousand. I searched carefully, but 

 found no other infected hair anywhere on this rootlet. The 

 proportion of infection in this case is as one to a thousand. 

 The rootlet examined was from a young Bur Clover seed- 

 ling growing in sandy soil in the laboratory. There must 

 have been a large number of Bur Clover bacteria in the 

 soil used, for it was taken from a spot where Bur Clover 

 throve last year and again this. Out of doors I fancy that 

 the number of root-hairs would be greater in the same sandy 

 soil than in the laboratory, for I watered my indoor mate- 

 rial, and that outside was watered only by the rains. The 

 number of root-hairs attacked is probably no greater, how- 

 ever. The proportion of one to a thousand is therefore 

 conservative. If, then, these bacteria are motile only 

 slowly, if at all, it is apparently the mere chance of a root- 

 hair's growing to or very near where the bacteria are which 

 makes infection possible. We may infer either that the 



1 This I estimated with considerable precision by means of a mechanical stage. 



