84 KNOT-ROOT DISEASE IN CUCUMBERS. 



The larvae -when liberated seek out a root of the beet and 

 bore into it, and here they live, causing the disease. When 

 they have entered the root they produce great swellings — 

 "galls," in fact — over their abode (fig. 33, c, d). The larva (fig. 

 33, b) that has thus entered the plant sheds its skin, and instead 

 of being elongated it becomes thicker in shape, ceases to move, 

 and lies with the bulbous patch formed over it. At this time 

 the sexes commence to appear distinct. If the thick motionless 

 larva is to become a male it ceases to feed, shrinks within its 

 old skin, and develops a thin new one, very like the puparium 

 stage of some insects. The newly-formed male worm inside 

 its old skin is more elongate — in fact, it resembles the typical 

 form of an eelworm. "When mature this male bores its way out 

 of the root and commences its search for a female. Should the 

 larva become a female, the development is much simplified. 

 The female develops by a simple distension of the body and 

 the formation of the female sexual organs. There is no pro- 

 cess of reformation as we observe in the male. When the 

 female is fully formed, the lump on the root ruptures and 

 releases the worm ; but she, on the other hand, does not re- 

 linquish her hold of the plant, to which she remains permanently 

 attached. 



The development from the egg to the adult takes about 

 five weeks. As many as six or even seven generations occur 

 in the year. This same worm attacks hops, producing the 

 disease called " Nettle-head " ; it is also found in wheat. 



Knot-root Disease in Cucumbers, i^c. 



" Knot-root " is a disease which affects cucumbers, tomatoes, 

 marrows, &c. It is caused by the eelworm Heterodera radicicola, 

 which produces swollen areas and knots of various sizes on the 

 fine and thick roots. The eelworms swarm in these galls, and 

 when the latter decay pass into the soil, which becomes infested 

 with them, ready to attack any fresh plants. This pest has 



