72 Bacterial Diseases of Plants 



on examination of the seed the cotyledons are found to have brown spots 

 or bacterial cavities at the centre of each. 



The parasite. P. seminum is a large rod motile by a single polar 

 flagellum. It stains well with the usual stains and is (ham positive. On 

 solid media it readily forms oval bodies which may occur strung together 

 in chains giving the appearance of beaded rods. These oval bodies are 

 thought by Miss Cayley to be involution forms which contain certain 

 particles, these stain as spores and are believed to be spores though their 

 germination has not as yet been observed. 



Host plants. At present the disease has been found only in Pisum 

 sativum, all varieties of which would seem to be susceptible to more or 

 less extent. The most susceptible varieties on the soil of the John Innes 

 Institution being Ne Plus Ultra and Duke of Albany. The dwarf earlies 

 Chelsea Gem and liittle Marvel are also specially susceptible, while 

 Sutton's Improved Petit Pois has proved on this soil to be least affected. 

 In this variety, however, only about 50 per cent, of healthy plants were 

 obtained. 



Distribution. The disease is believed to be fairly common, at least 

 throughout the South of England, and to be steadily on the increase. 

 Miss Cayley reports that it is each year becoming more difficult to find 

 seed for experimental purposes which can be relied upon to be free from 

 this disease. 



Potato Scab. 



The well-known corky patches or scabs on the skin of the potato are 

 the result of the response of the cortical cells of the tuber to the stimulus 

 of an invading organism and not, as has been frequently stated, the 

 result of mechanical injury or of the attack of wire-worms or other- 

 animal parasites. In its defence against the parasite the tissue of the 

 potato is so far successful as to limit the attack to a few of the outer 

 layers of cells only. Successive layers of cork are formed as the organism 

 penetrates into the tissue and these may extend to a depth of half a 

 centimetre, but the internal storage cells remain unaffected. Bolley(r>) 

 isolated a small bacterium from scabbed potatoes in Indiana and with it 

 produced scabs by artificial infection. Later however (7) he expressed 

 doubt as to whether this organism is responsible for the disease under 

 natural conditions, since, in subsequent efforts, he was unable to confirm 

 his original work, but had no difiicultv in showing that the usual cause 

 of the disease is an organism which was isolated by Thaxter(iS) and 

 descriJK'd as Oospora scabies. The disease was included anu^ngst those 



