E, J. Welsford 43 



Owing to the terrible destruction of hyacinths in Holkind the 

 "ring disease," as it is called, has been thoroughly studied in that 

 country, more especially by de Mann (7) and by Ritzema Bos (2,3). The 

 latter believes that the slight differences which exist between the 

 eelworms parasitic on rye, hyacinth, onion and numerous other plants 

 are not of specific value but are due to environment. He considers 

 that the eelworm present in these plants should be called Tylenchus 

 devastatrix, Kuhn, and he publishes a diagnosis of the species with a 

 detailed description and numerous figures (iii. pp. 185, 220). As the 

 characteristics of the eelworm found in Narcissus bulbs in England 

 agree exactly with the diagnosis of Tylenchus devastatrix as given by 

 Ritzema Bos, there seems no doubt of the correctness of the identification 

 hete put forward. 



According to Ritzema Bos (iii. pp. 232-234), Tylenchus devastatrix 

 attacks a variety of cultivated and wild plants, and though it is capable 

 of passing from one to the other it does not do so easily. He con- 

 cludes that there are physiological varieties of the species and says: 

 "II paraitrait done qu'on peut admettre comme regie, que le Tylenchus 

 devastatrix se fixe dans les plantes dans lesquelles les ancetres ont vecu 

 depuis plusieurs generations, de preference a d'autres especes de plantes; 

 et que, toutes choses egales d'ailleurs, il prefere ordinairement la plante 

 ayant une parente etroite avec celle dans laquelle vivaient les genera- 

 tions precedentes, a celle qui en est plus eloignee dans le systeme. 



" J'estime qu'on pourra trouver dans ce qui precede la clef de bien 

 des choses, inexpliquees jusqu'ici relativement a I'apparition et a la 

 disparition des maladies vermiculaires dans nos cultures. 



" II resulte d'ailleurs de ce qui a ete traite jusqu'ici dans ce chapitre, 

 que, si au point de vue morphologique il faut considerer comme une espece 

 unique les Tylenchus des jacinthes, ceux de loignon, ceux du seigle, 

 du sarrasin, du trefle, etc., et ceux de la mousse Hypnum cupressiforme, 

 il existe neanmoins des differences physiologiques entre les dift'erentes 

 Tylenchus, selon que leurs ancetres ont vecu durant un grand nombre 

 de generations, dans telle ou telle plante." 



The differences between the eelworms of hyacinths and of onions 

 indicated by Ritzema Bos suggest a comparison with the "biologic 

 forms" described for various parasitic hosts. It is to be noted, however, 

 that Ritzema Bos ascribes to the eelworms merely preferences for their 

 original host while the biologic forms are normally confined by physio- 

 logical differences to their particular host or set of hosts. The physip- 

 logical differences between the onion and hyacinth "forms ' of eelworm, 



