Bacteria in their Relation to Vegetable Tissue. 23 



by a consideration of the phenomena connected with certain fungal 

 diseases, as in the case of Cystopus Candidas, which, according to 

 De Bary, can only successfully infect the host-plant when it enters 

 the young cotyledons of the plant. Hypoderma macrosporon is 

 only able to gain entrance into the pine through the young pine 

 cones. 



This law usually holds in connection with the bacterial plant- 

 diseases. Most of them are more virulent in their course when they 

 attack young and undeveloped hosts, and not a few are apparently 

 inhibited after the tissues have reached a certain stage of maturity, 

 as in the case of the oat-disease of Galloway and the pear-blight. 



CAUSES OF IMMUNITY AND RESISTANCE. 



Having cited special cases illustrative of resistance and immunity, 

 we may now turn to the consideration of the possible factors which 

 are able to cause these conditions. To this difficult problem we can- 

 not hope as yet to give any definite and conclusive answer. The rapid 

 progress which has been made in this department of animal biology 

 within the past five years indicates how vast and complicated the 

 question is in all its bearings. The accumulating data which have 

 already been collected have, however, contributed much to a better 

 conception of the problem of immunity, and lead us to believe that 

 any experimental consideration of this subject in relation to plants, 

 even though negative in its results, is not entirely without value. 



The attempt has been made in the previous pages to show that 

 there are two factors at work in the struggle of the plant with its 

 parasitic enemies. We shall not, however, attempt to prove that the 

 phenomena which are considered as resistance and immunity are 

 brought about through the action of separate and distinct forces. It 

 is possible, and quite probable, -that the conditions which produce 

 one factor may also be the cause of the other. For instance, in 

 animal biology, Wyssokowitsch determined that certain saprophytes, 

 as well as some parasites, disappeared completely when introduced into 

 the blood of a living rabbit. Nuttall determined that the cause which 

 destroyed saprophy tic as well as pathogenic organisms was the same, 

 namely, the germicidal property of the body fluids. Although the 

 result was brought about through the operation of the same cause, 



