BoT.— Vol. II.] PEIRCE— ROOT-TUBERCLES. 319 



the number of bacteria. The cells with few or no bacteria 

 receive more non-nitrogenous food than they consume. The 

 excess they deposit in solid form as starch. The cells with 

 many bacteria presumably receive at least as much non-ni- 

 trogenous food; but either they themselves or the bacteria 

 in them consume this so that there is no excess to deposit. 

 From the cells toward the center of the tubercle the new 

 cells formed by the tubercle meristem are infected by means 

 of infection threads running fairly straight toward and into 

 the daughter-cells of the meristem (see fig. 10). This figure, 

 magnification two hundred, was drawn from a thin micro- 

 tome section of a young tubercle, and is colored as nearly 

 as possible like the cells of the preparation. The prepara- 

 tion was stained, as previously described, by Flemming's 

 triple stain, Gramm's iodine solution being used after the 

 anilin gentian violet in order to differentiate the strands of 

 bacteria. The cell walls are drawn in black, though they 

 were only very faintly stained and of course were not black. 

 The C3'toplasm is brownish yellow from orange G., the 

 nuclei a somewhat deeper shade of the same color, the 

 nucleoli red from anilin safranin, the infection threads pur- 

 ple from anilin gentian violet. In a fresh preparation, 

 whatever starch grains are present in the section are stained 

 the usual color by the iodine, but this color is fugitive. The 

 arrow to the left in this figure (fig. 10) shows the direction 

 in which the tubercle meristem lies. It is evident from this 

 figure that the infection threads run very definitely toward 

 the new cells formed by the merismatic layer. Since all 

 the food of the tubercle cells comes from the opposite direc- 

 tion, from the root, there must be some other directive 

 influence than this food to cause the infection threads to 

 grow so definitely toward the tip of the tubercle. This 

 influence must come from the cambium or from its daugh- 

 ter-cells, and must consist in the substance or the products 

 of these cells rather than in the food supplied to them. If 

 we are to assume any chemotactic influence, it must be 

 exerted by some diffusing substance or substances and 



