i6 



not mean that nematodes are always the agent which induces these 

 diseases, — poor drainage or ventilation, improper temperature or 

 fertilizers, and a hundred other things may serve to weaken the 

 plants and stop their growth, thus leaving them an easy prey to 

 disease, — but we do believe that nematodes are at the bottom of 

 much more trouble with plants than is generally suspected. 



Another secondary result of nematode attacks is worth consider- 

 ing. In examining roots which are badly infested we find not only 

 the worms of this particular species but also other kinds of nema- 

 todes, other low animal organisms, fungi, and bacteria, forms which 

 have no power to attack the healthy root but which come in after the 

 plant has been weakened and its root partly destroyed, and no doubt 

 aid considerably in hastening its death. Thus the injuries caused to 

 plants by Heterodera radicola are of three kinds ; first the small 

 direct injury by the worm feeding on the substance of the plant ; 

 second, and most important, the indirect injury brought about by the 

 interference with the vital functions of the plant on account of the 

 abnormal growth ; and third, secondary effects as described above. 



Description of Free-Living Nematodes. 



A typical nematode of the free-living, harmless class is shown in 

 plates I. and II. This is a form found in decaying roots which had 

 been killed by Heterodera. It is a species of Rhabdites. The ani- 

 mal originates from an egg, (PI. I., fig. i.) which is of a noval shape, 

 about .07 mm. (-gly of an inch) in length and half as wide, and con- 

 sists of a membranous covering inclosing a mass of granular proto- 

 plasm and fat globules. After being impregnated the contents of 

 the egg divide into two parts (fig. 2) and then by continual division 

 and development as shown in figs. 1-12, develop gradually into an 

 elongated structure which assumes the form of a young worm, 

 doubled up several times in the egg membrane. W'hen fully devel- 

 oped it bursts the membrane and is discharged into the water or 

 earth or wherever the mother may be. In this particular species the 

 young are born alive. In others the eggs are discharged as soon as 

 mature or when the young worm is partly developed, completing 

 their development outside the mother. The newly hatched worm 

 (fig. 13) is a minute elongated organism about .3 mm. (J^ of an inch) 

 in length, tapering to a rounded end at the head and a pointed tail 

 behind. Its structure is quite simple. The body wall is composed of 



