BOT.-VOL. II.] PEIRCE— ROOT-TUBERCLES. 325 



2. The proportion of root-hairs infected to the total num- 



ber formed is small, in one case computed to 

 be I : 1000. 



3. Given the contact or close proximity of tubercle bac- 



teria with the root-hairs, infections may take place 

 in great numbers simultaneously, at least when the 

 roots are very young, 



4. Infection of the root may be resisted by cutting off the 



infected ends of the root-hairs. 



5. The tubercle bacteria enter and infect a root-hair by 



softening or dissolving a small portion of the wall 

 and moving or growing through this. There is no 

 evidence that they usually enter through broken 

 root-hairs, and the curvatures of infected root-hairs 

 are evidence against these hairs having been broken 

 at any time. 



6. The infection thread grows fairly straight, being 



chemotropically attracted, through the cortical paren- 

 chyma, from the root-hair to the layer of cells next 

 outside of the central cylinder of the root. 



7. The tubercles originate only endogenously and from 



the same layer as gives rise to lateral roots. We 

 may therefore conclude that the tubercles are mor- 

 phologically lateral roots, though greatly modified by 

 the influence which caused them to be formed. 



8. Tubercles form only as the result of stimulation by 



bacteria. Do lateral roots form as the result of 

 internal causes or of external stimuli? 



9. The growth of the tubercle is apical, the daughter-cells 



of a bowl-shaped terminal meristem constituting the 

 growing part of the tubercle. There is little or no 

 secondary growth in thickness. Because of this, 

 the conducting tissues do not keep pace with the 

 growth of the tubercle. The growth of the tubercle 

 is correspondingly limited. 

 10. Tubercles are largest and most numerous near the 

 surface of the soil. It is possible that perennial 

 Leguminosas form few if any tubercles after their 

 roots have grown deep into the soil. 



