18 DISEASES OF CROPS. 



during the pairing season that they become parasitical, 

 for then they require an abundant nitrogenous food in 

 order to develop their sexual organs ; and hence they are 

 found in the sexual state only within the tissues of clover 

 and other plants. During the winter the sexless worms 

 appear to live or hibernate in moist earth (Kiihn). The 

 female multiplies with great rapidity so long as the store 

 of food lasts. It appears that their power of fecundity is 

 in direct proportion to the quantity of living organic 

 nitrogen they are able to obtain. When the food is ex- 

 hausted the last hatched young wander away and attack 

 other plants, or assume the " dormant " condition until 

 another season. The eggs are always laid within the 

 host-plants. These give rise to larvae which are " more 

 blunt at the extremities than the mature eelworms." 



The clover eelworms are possessed of very great tenacity 

 of life. According to Dr. C. Devalue (" Recherches sur 

 I'Anguillule du bl6 nielle ") certain species of Tylenchus 

 may be exposed to —4° !P. " for several hours, without 

 destroying their vitality. About 148° F., however, is the 

 maximum of heat in which they can exist." Eelworms 

 appear to be able to retain their vitality for several years ; 

 and Dr. Kuhn {Ueber die Wurmkrankheit des Roggens) 

 found that Tylenchus devastatrix retained its life for two 

 years in a state of complete desiccation, and much longer 

 in damp earth. 



" With respect to infested clover plants . . . some 

 of the eelworms would remain in the leaves and stems left 

 in the plants after the first and second ' cuts ' had been 

 mown. These, together with any that may have escaped 

 into the ground, are buried by the plough, and are brought 

 up again by a future ploughing in course of time to the 

 reviving influences of air and light. It is most probable 



