328 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



EXPLANATION OF PLATE XXIX. 



Most of the figures were drawn with the aid of a Leitz drawing prism 

 ocular, the rest with an Abb6 camera lucida. 



Fig. I. Two root-hairs of Bur Clover infected by tubercle bacteria, 

 showing the characteristic bending at the point of infection; x 50. 



Fig. 2. The lower of the two root-hairs of fig. i. The mass of bacteria 

 in the concavity of the coil and the infection thread running from this point 

 through the hair; x 300. 



Fig. 3- Another infected and coiled root-hair with the infection thread 

 growing close to the nucleus of the hair; x 300. 



Fig 4. A section showing the course of the infection thread from the 

 base of the hair (r. h.) to the layer of cells which gives rise to the tubercle, 

 the same layer which, under other conditions, would give rise to a lateral 

 root; X 360. 



Fig. 5. A section showing a young tubercle (left) and a young lateral 

 root (right) developing from the same layer; x 20. 



Fig. 6. The same section enlarged. In the lateral root (right) differen- 

 tiation of tissues is already taking place, but not in the tubercle (left): x 360. 



Fig. 7. Longitudinal section of a tubercle, young and still growing. 

 The largest part of the tubercle is composed of bacteria-containing cells. 

 Outside of this mass is the comparatively thin layer in which are the small 

 and scattered vascular bundles. Beyond this is the protective tissue, pow- 

 dery on the outside, which is continuous with the cortex of the root. At 

 a-b is the tubercle meristem, which forms daughter-cells both forward and 

 backward, as does the growing-point of a root; x 35. 



Fig. 8. A typical infected tubercle cell, thin-walled, with the cytoplasm 

 surrounding a large single vacuole, and the nucleus reduced to a small lumpy 

 plate. Cytoplasm stained with orange G., but the color obscured and dulled 

 by the purple of the bacteria stained by anilin gentian violet. The riucleus 

 is purplish from the mixture of anilin safranin and anilin gentian violet, a 

 wholly different color from that of a healthy nucleus stained and differentiated 

 by Flemming's triple stain and Gramm's iodine solution. Intercellular 

 spaces are shown; x 660. 



Fig. 9. Tubercle cell containing only small number of bacteria (at B) and 

 enclosing some starch-grains (S). The nucleus has degenerated only some- 

 what and the vacuolization of the cytoplasm has not progressed far; x 820. 



Fig. 10. Section of a tubercle, part of tubercle shown in fig. 7, near the 

 meristem. The direction in which the meristem lies is indicated by the 

 arrow at the side. The section was stained by Flemming's triple stain and 

 differentiated, after the anilin gentian violet, by Gramm's iodine. Thus the 

 infection threads are cleariy brought out. Note the course of the infection 

 threads, definitely toward the tubercle meristem and generally toward the 

 nuclei of the cells entered; x 200. 



Fig. II. One cell from fig. 10 (nearly in the center of the figure) showing 

 the solid infection strand (zoogloea) in which the separate bacteria can be 

 distinguished; x 1000. 



Fig. 12 and 13. Tubercle cells showing the infection threads growing in 

 definite direction, generally toward the nuclei of the cells; x 820; 



Fig. 14, a-d. A series of cells from the meristem backward in a longitu- 

 dinal section of a tubercle. The section is stained by Flemming's triple 

 stain, but not differentiated by Gramm's iodine, hence infection threads do 

 not show. In fig. 14 a, two meristem cells show at x-y, with daughter- 

 cells both forward and backward. Z= already infected and degenerating 

 cell. In 14 b, degeneration of the cell and especially of the nucleus is shown 

 by cells c, i, h. Degeneration of the nucleus, obscuring of the color of 

 the cytoplasm by the many bacteria in it, and the formation of the char- 

 acteristic vacuole shows in 14 r, especially the two upper cells. Fig. 14^ 

 shows the same still more markedly, especially the uppermost cell. In this 

 series the effect of bacterial infection of the cells is clearly exhibited; 14 a, x 

 270; 14 b-d, X 300. 



Fig. 15. Division of an already, though recently, infected daughter-cell 

 of the tubercle meristem; x 300. 



