THE CAUSAL PARASITE. 39 



Cobb expresses the same opinion based on his observations in New 

 South Wales/ 



For the North, where root-knot is mostly confined to greenhouses 

 and hotbeds and their vicinity, perhaps one of the chief sources of 

 infection is the soil that is thrown out of these beds at the end of the 

 season. This soil, if infested, will spread the disease in the imme- 

 diate vicinity, especially if it be put near some manure pile or compost 

 heap which keeps the ground damp and warm during the winter. 



EFFECT ON THE HOST. 



The effect upon the root of the presence within it of the young 

 nematode is usually the hypertrophy of some of the tissues. The 

 parenchyma cells become abnormally large and multinucleate,^ 

 sometimes only a few, at other tunes a great many cells being involved 

 in this hypertrophy. This abnormal enlargement of the cells leads 

 to a displacement of the various tissue elements, so that the tracheary 

 cells and vessels become separated and also show lateral displace- 

 ment and often much distortion. Often in bad cases individual cells 

 of a tracheary nature will occur entirely separated from others of the 

 same kind. The amount of hypertrophic enlargement of the root 

 depends upon the host on the one hand and upon the number of 

 nematodes entering the root in the same vicinity on the other. In 

 some roots the swelling is barely noticeable and is so small that as 

 the female nematode enlarges she eventually is inclosed in the root 

 only by the narrow anterior third of the body, the remainder bemg 

 entirel}^ external, in this particular showing great similarity to the 

 sugar-beet nematode, whose galls are always of this nature. More 

 often, however, the hypertrophy is so pronounced that the mature 

 female is entirely concealed or reaches the surface only at the extreme 

 posterior portion of the body. If many nematodes are present in 

 the same general region of a susceptible root, the gall may be many 

 times the normal size of the root (PI. II, fig. 2). These galls are at 

 first of soft tissues, but in some woody plants, the European elm, for 

 example, some of the hypertrophied cells become lignified, inclosing 

 the female nematode in a woody prison from which in all probability 

 the larvae would be unable to escape should egg laying continue 

 after the lignification has begun. The structure of such a gall is like 

 that of the burls that often occur on various trees. 



A very frequent phenomenon, but one that is by no means uni- 

 versal or characteristic of any one group of plants, is the formation 

 of numerous lateral rootlets above the gall. This is doubtless due 



1 The wTiter's attention has been called to the fact that in certain of the irrigated districts of the West 

 this nematode has become a very serious potato trouble. On one occasion several carloads of potatoes 

 were rejected on account of being infested with it. 



2 Tischler, 1902. 



217 



