1915] 



SMITH BACTERIAL DISEASES OF PLANTS 



385 



In Italy, the olive tubercle due to Bacterium Savastanoi 

 has been observed to begin very often in wounds made by hail- 

 stones. In South Africa, crown-gall is said to be disseminated 

 in the same way. In this country and also in Sumatra, Bac- 

 terium Solanacearum enters the plant more often than other- 

 wise through broken roots. A tomato or tobacco plant with 

 unbroken roots will thrive in a soil deadly to one that has 

 been root-pruned. I have myself observed this. We may 

 suppose that substances attractive to the particular bacteria 

 diffuse into the soil from the broken roots, following which 

 they enter the plant. Eesistant plants may be supposed to 

 diffuse indifferent or repellant substances. All infections 

 must be chemotactic. 



More interesting perhaps are those diseases which begin in 

 natural openings, i. e., in places where the protective covering 

 of the plant gives place to special organs such as nectaries, 

 water-pores, and stomata. 



All the pome fruits subject to fire-blight are liable to blos- 

 som infection. The bacteria multiply first in the nectaries of 

 the flower, passing down into the stem by way of the ovary 



and pedicel. Blossom blight of the pear is a very conspicuous 

 and common form of the disease as everybody knows. Thou- 

 sands of blighted blossom clusters may be seen in any large 

 orchard subject to this disease. 



In the black rot of the cabbage due to Bacterium campestre, 

 the majority of the infections begin in the water-pores. These 

 are grouped on the margins of the leaf at the tips of the ser- 

 ratures. From this point the bacteria burrow into the vas- 

 cular system of the leaf and so pass downward into the stem 

 and upward into other leaves. 



In the black spot of the plum, almost or quite all of the 

 infections are stomatal. A large proportion of them are also 

 stomatal in the leaf-spot of cotton, and other leaf-spots. 



TIME BETWEEN INFECTION AND APPEARANCE OF THE DISEASE 



As in animal diseases, the period of latency may be very 

 short or surprisingly long. Some time must be allowed the 

 parasitic organism to multiply inside the plant before it does 



