336 PLANT DISEASES 



examination, that the organism concerned differed from 

 anything previously connected with plant disease. 



Woronin, a Russian botanist, was the first to clearly 

 work out the life-history of the organism causing the 

 disease, his attention being called to the subject by the 

 offer of a reward by the Russian Government for the dis- 

 covery of the serious disease affecting the cabbage crop in 

 that country. 



The disease is contracted in the first instance by spores 

 present in the soil, which enter the living cells of the root 

 of the host. When once inside a living cell, the parasite 

 does not form a mycelium as in the fungi, but a glairy 

 mass of protoplasm called a plasmodium, which slowly 

 alters its form from time to time, and constitutes the 

 vegetative condition of the parasite. Cells containing 

 plasmodium increase enormously in size, and furthermore 

 the plasmodium passes from one cell to another, devour- 

 ing their contents. After remaining in the vegetative state 

 for some time, and producing the swellings in the root 

 alluded to, the plasmodium undergoes certain changes, 

 and eventually becomes resolved into myriads of exceed- 

 ingly minute, round spores, which on the decay of the 

 root are liberated in the soil. What takes place after the 

 spores are set free, as to whether they germinate, form a 

 plasmodium that exists as a saprophyte for some time, is 

 not known ; but it is known with certainty that if cabbages, 

 turnips, or other allied plants are sown or planted in soil 

 that has produced diseased plants the previous season, or 

 even two or three years previously, such plants will become 

 diseased. 



PREVENTIVE MEANS. From what has been stated, it 

 will be seen that it is simply courting disaster sowing or 



