220 ZOOLOGY. 



throughout life, but the adult females of the latter 

 swell out considerably and become lemon- or pear- 

 shaped. 



The Stem Eelworm (Tylenchus devastatrix). 



Length one-thirtieth to one-fifteenth of an inch long, 

 usually of intermediate size ; the two sexes of approxi- 

 mately equal length. Live and reproduce in various 

 cultivated plants {e.g., rye, oats, stored onions, 

 hyacinths, buckwheat, potatoes, clover, fuller's teasel) 

 and wild plants (e.g., Poa annua, Anthoxanthwm 

 odoratum, Dipsacus silvestris, Polygonum, persicaria), 

 but not to the same extent in all. It must also be 

 added that eelworms of which the progenitors have 

 developed for a considerable number of generations in 

 the same plant, are not easily transferred to another 

 kind of plant, or at any rate do not multiply 

 vigorously there. Eelworms, of which the ancestors 

 have lived for many years exclusively in rye, or 

 alternately in rye and buckwheat, do not readily pass 

 over to seedling onions, and first only reproduce in 

 them to a small extent. The eelworms live only in 

 stems, branches, and leaves, never in roots. In the 

 places where they have penetrated the plant tissues 

 an abnormal growth in thickness of the parts involved 

 takes place, while the growth in length is either 

 much diminished or even entirely stopped. Also the 

 chlorophyll disappears sooner or later from the 

 attacked spots, and rapid death generally ensues. 

 Since only those parts of an organ which are in- 

 habited by a large number of eelworms swell much, 

 it is obvious that cracks are often developed in the 

 stems and leaves concerned. It is further easily seen 

 that the species and constitution of the infested plants 

 wUl have an important infiuence on the progress of 

 the disease caused by the eelworms. But since several 

 generations of this parasite succeed one another 



