Bacteria in their Relation to Vegetable Tissue. 33 



account for the distribution of bacteria in the tissue. It seems to be 

 correlated much more closely with the actual growth of the micro- 

 organisms. 



7. The facts already determined relative to the ability of sapro- 

 phytes to thrive in plant-tissues throw some light on the question 

 of the normal presence of bacteria in healthy plants. A large 

 number of cultures made from the inner tissue of healthy plant stems 

 revealed no bacteria, but where stems were wounded, even by a 

 needle-prick, the bacteria on the surface were able to enter and live 

 for a long time. Thus it is possible that bacteria may enter through 

 lesions so small as to escape notice, or they might even live in the 

 tissue after the wound had healed over. 



8. With bacteria, not adapted for growth in plants, I have been 

 unable to prove that they could enter the plant where the epidermal 

 tissue was known to be intact. In the case of parasitic species on 

 plants, they sometimes effect an entrance into tissues without the 

 intervention of wounds of any sort. 



9. The actual method by which saprophytes are able to spread in 

 plant tissues has not been satisfactorily determined, but it seems that 

 the cellulose wall undergoes a change that renders it permeable to 

 the bacteria. Those forms causing a pathological condition in the 

 plant spread, in many cases, by means of the fermentative and 

 destructive power they possess. 



10. The phenomena, heretofore regarded as immunity of plants 

 from micro-organisms, present two phases so distinct in their action 

 that it seems proper to separate them to a certain degree. The 

 exemption of plants from bacteria in general is due to what may be 

 termed the resistance of the plant, while the more restricted term, 

 immunity, is reserved for the ability of a certain group of plants to 

 be refractory toward a disease germ that is able to cause a patho- 

 logical condition in closely allied forms of plant life. No hard and 

 fast limits can be drawn for the immunity of plants, as this condition 

 varies in each disease. The causes which bring about this ability of 

 the plant to repel not only bacteria in general, but those toward 

 which it is somewhat susceptible, are various. 



In the case of immunity, physical causes, such as the epidermal 

 and cortical resistant tissues, matured and thickened cell walls of the 

 inner tissue, exclusion by gummy exudates, etc., are the leading 



