230 ZOOLOGY. 



lure-plants must be sown as thickly as practicable, so 

 that the soil may be penetrated by as many slender 

 rootlets as possible. After these plants have been 

 dug up, a second lot should be grown, since all the 

 eel worms will not have attacked the first lot ; and it 

 is even advisable to grow a third batch. Kiihn used 

 as lure-plants the various kinds of cabbage, also 

 summer rape (Brassica rapa), since this plant has a 

 great attractive power for the beet eelworms, and can 

 hold a large number of them in its numerous, much 

 branched rootlets. I cannot go into all Kiihn's re- 

 searches here, and will only mention the following. 

 In the course of the year 1880 part of a beet-sick 

 field had three successive crops of lure-plants grown 

 upon it, each being dug up from thirty to forty days 

 after sowing. The field was ploughed in autumn, 

 suitably manured the next spring, and sown with 

 beet in the middle of April. The other part of the 

 piece of land was treated in exactly the same way, 

 except that no lure-plants were grown upon it. A 

 diflference was very soon seen between the two plots, 

 and there was a very great difference at the time the 

 crop was ready to be gathered in. The plants on all 

 parts of the first plot were in a flourishing condition, 

 but those on the second plot were in many places 

 either killed outright or else small and misshapen. 

 The crop succeeding the lure-plants was three times 

 as great as it had been before, and almost equal to 

 that of a healthy field. Later on, Kiihn made an 

 important discovery ; he found that larvae which have 

 reached the thickened motionless stage, depicted in 

 Fig. 134, 5, require a considerable amount of food to 

 keep them alive, and enable them to develop further. 

 If the plants are disturbed in such a way as to kill 

 the rootlets containing the larvae in this stage, these 

 are unable to develop any further. Kiihn caused a 

 kind of horse machine to be made, adapted for rapidly 

 destroying the lure-plants in the fields. For further 

 details, his original memoirs may be consulted. 



