192 NATURAL SCIENCE. March, 



of parasitic nematodes present in th-e hop roots. The exact part 

 which each plays in the disease it is impossible to decide without 

 carrying out a series of infection experiments, and such are now in 

 progress. Neither species finds its way into the stems or leaves of 

 the hop ; and, judging from their position, manner, and rate of 

 development, the peculiar changes observed in the leaves are no 

 doubt to be attributed to the secretions of the worms being carried 

 there in the ascending current of sap. 



The Tylenchus is one of the causes of clover " stem-sickness " and 

 " tulip-root " of oats, and is also met with in stems of buckwheat, rye, 

 hyacinths, onions, and several other plants. The Heterodera schachtii is 

 the cause of "beet-sickness," and is also known to attack and live upon 

 the roots of all cereals, and practically all cruciferous crops, as turnip, 

 cabbage, kohl-rabi, mustard, etc. Lately some leguminous crops 

 have been attacked. Potatoes and all Compositae (in which Order the 

 artichoke, lettuce, and salsify are the chief food-plants) seem to be 

 free from injury by this parasite. 



With regard to remedies, it may be remarked at the outset that 

 there is no known method of exterminating these nematodes from the 

 soil when once they have become established there, and consequently 

 it becomes imperative to call attention to the necessity for dealing 

 summarily with the pest in the earliest manifestations of this new 

 disease. 



Thousands of experiments have been made on the Continent 

 with a view of eradicating the Heterodera from the sugar-beet fields, 

 but without success. The most satisfactory means of keeping it 

 within bounds was suggested by Kiihn after his researches in 1881, 

 and his plan has so far yielded the best results, although it is no cure. 

 He finds that the larvae, after entering the roots of plants, require at 

 one stage of their development considerable quantities of food. If 

 this supply is cut short, the parasite dies before reaching maturity 

 and before the fertilisation of the females has taken place. <; Trap- 

 plants " are grown, the most suitable being summer rape, on account 

 of its extensive root-system, easy infection, and rapidity with which it 

 dies when disturbed. After the larvae have entered the rootlets and 

 reached a certain stage of growth, the crop is destroyed, and with it 

 all the immature parasites. A repetition of this process three or four 

 times during the summer cleans the ground of most of the parasites, 

 and a remunerative crop of beet can then be taken off in the ordinary 

 course of cultivation for some time before the " sickness " shows 

 itself again to any serious extent. To reduce the expense of idle 

 ground, a rapidly-growing crop of potatoes is taken off in midsummer. 

 It is obvious, of course, that such a process is not applicable to hop- 

 gardens where the plants remain on the ground several years, but 

 some such scheme must be adopted in order to clean the ground after 

 grubbing a badly attacked garden. There is hope in a properly 

 selected rotation of crops for diminishing the number of nematodes in 



