54 



BACTERIA IN RELATION TO PLANT DISEASES. 



INFECTION THROUGH NATURAL^ OPENINGS. 



Inasmuch as certain scientific men of excellent reputation have doubted the possibility 

 of any spread of bacteria through living plants except by wound-infections, it appears to be 

 worth while to summarize somewhat carefully what is known of bacterial infections in the 

 absence of visible wounds. The greater portion of the plant-body in the higher plants is 

 well protected by a cutinized epidermis or by a still more resistant cork-layer, through 

 which bacteria would find it difficult to make their way. There are, however, many natural 

 openings, wholly unprotected places, and it is through these that infection takes place. 



Fig. 8.* 

 Nectarial infection. — The best-known case of infection through the nectaries is that 

 of the pear, which is commonly attacked in this manner by Bacillus amylovorus. The 

 organism, brought to the plant by bees and other nectar-sipping insects, multiplies enor- 

 mously in the floral nectaries, which are blackened and killed. From this nidus the blight 

 bacillus passes into the ovary and down the pedicel of the flower into the stem, which 

 blights in turn. In moist, warm springs the progress of the floral infection is rapid, and it is 

 not at all uncommon to find thousands of bHghted blossom-clusters in a single large orchard. 

 The organism attacks vigorous young shoots in the absence of blossoms, but infection by 

 way of the nectaries is extremely common. Blossom-blight was observed by Dr. Arthur, 

 who called attention to it in several publications, now 20 years old, but he does not appear to 



*Fio. 8. — Twig of Magnolia fraseri, sliowiug frost injuries on five immature leaves; other two leaves developed 

 after frost and are free from injury. Grounds of U. S. Dept. of Agriculture, May :?, 1905. The light frost occurred in 

 April and none of the leaves showed the usual aspect of frosting. 



