GEORGE K. K. LINK 603 



crown-galls. The report that legume roots become immune to invasion by less viru- 

 lent strains of bacteria after infection with more virulent strains of nodule bacteria 

 is interesting and deserves reinvestigation. There are many instances of recovery of 

 individual plants or their organs from attack by bacterial pathogens. These may be 

 due to cessation of environmental factors favorable for the host, and not to an ac- 

 quired immunity. Whether the self-limited spots and cankers that characterize many 

 bacterial diseases are partly or wholly due to acquired immunity is a possibility which 

 should be investigated. 



RELATION OF PATHOGEN TO HOST TISSUES AND CELLS 



Only one case of penetration of cell walls and intracellular life of a bacterial patho- 

 gen has been established. This occurs in the legume-nodule-producing bacteria. It 

 was contended by Smith' that Bad. tumefaciens was another illustration of an in- 

 tracellular schizomycete. However, the later findings of Riker- and of Robinson and 

 Walkden^ to the effect that Bad. tumefaciens invades mainly through the intercellular 

 spaces, and occasionally through the vascular system, is at variance with the earlier 

 reports of Smith. 



In a few diseases which are known as "tracheo-bacterioses," the pathogens are 

 very abundant in the lumina of vessels in the early stages of the disease. Very soon, 

 however, the organisms invade the surrounding parenchymatous tissues. Unless they 

 are directly introduced into the vessels by injuries — for example, insect bites as in bac- 

 terial wilt of the cucumber — they must pass through parenchyma before they enter 

 the vessels as in black rot of cabbage. By far the greater number of bacterial path- 

 ogens Hve in the intercellular spaces of the plant. 



Route of entry. — Some bacterial pathogens at times, or consistently, have definite 

 routes of entry. B. amylovorus may invade through the unprotected surface of the 

 flower nectary, though it also invades any young tissue through wounds; Bact. cam- 

 pestre invades through the hydathodes and progresses through the vascular system, 

 though it also enters through wounds; many pathogens enter through the stomata or 

 through wounds; some enter principally through lenticels {Actinomyces scabies); and 

 a great number enter through wounds. The legume-nodule bacteria enter through 

 root hairs. Brown" has recently shown that Bact. malvaceanim gets its first foothold 

 in young seedlings in the injuries made in the cotyledons as these emerge from the seed 

 coat and soil. Insects and cultural practices play an important role in getting 

 bacteria past the effective defense mechanism of the epidermal or cork layers of 

 plants. 



The mechanism of tissue invasion. — Following the early critical work of Spiecker- 



' Smith, E. F.: "Appositional Growth in Crown Gall Tumors and in Cancers," J. Cancer Re- 

 search, 7, 1-105. 1922; "Mechanism of Tumor Growth in Crown Gall," /. Agr. Research, 8, 165-86. 

 191 7; Smith, E. F., Brown, N. A., and Townsend, C. O.: loc. cit. 



^ Riker, A. J.: "Some Morphological Responses of the Host Tissues to the Crown Gall 

 Organism," /. Agr. Research, 26, 425-36. 1924. 



3 Robinson, W., and Walkden, H.: loc. cit. 



■* Brown, J. G.: The Influence of Alkaline Soils on the Prevalence of Angular Leaf Spot in Pima- 

 Egyptian Cotton. Thesis, Botany Department, University of Chicago, 1925. 



