30 ROOT-KNOT AND ITS CONTROL. 



Except anterior to the digestive portion of the alimentary canal 

 the body cavity is small. There are no signs as yet of the repro- 

 ductive organs, nor can the sexes be distinguished. 



The larvse are actively motile, but not so active as many of the free- 

 living forms. Unlike the larvte of some nematodes parasitic upon 

 plants, for example, Tylenchus tritici,^ T. dipsaci,^ and a species of 

 Aphelenchus discovered by Dorsett ^ on the violet and studied by the 

 writer, the larvae of Heterodera radicicola are not very resistant to 

 unfavorable conditions. The other nematodes mentioned are unin- 

 jured by desiccation for long periods, by cold, many acids, etc. Thus, 

 the wheat nematode has been revived after having been left dry for 27 

 years. The Aphelenchus referred to remained alive in kerosene 

 emulsion for two days in contact with a drop of k9rosene. Osmic- 

 acid fixatives killed it but slowly, as was true of cliromic acid, mer- 

 curic chlorid, and other strong poisons. On the other hand, the 

 larvae of Heterodera radicicola, although able to remain alive in water 

 for a few days, soon die and decay, although damp or wet soil, pro- 

 vided the air supply is good, is favorable to their existence. Drying 

 out is usually fatal to them in a comparatively short time. 



The larv^ of the root-knot nematode are able to remain alive in the 

 soil for months without entering upon a parasitic existence. The 

 waiter has been unable, however, to find any evidence that they take 

 any nourishment from the soil; at least they undergo no development 

 mitil they enter the roots of some plant, for if the soil be kept free from 

 vegetation for two years they all die. Even one year without food is 

 sufhcient to kill large numbers of them. 



In the normal course of development the larvae, having encoun- 

 tered a root, seek its growing point and batter their way into it by the 

 aid of the buccal spear (PL I, fig. 17). They then take up a position 

 entirely within the root and parallel to its longitudinal axis, the 

 anterior end pointing away from the root tip. This position may be 

 in the plerome, or perhaps as frequently, if not more often, in the 

 periblem. In the former case the nematode lies witlim the central 

 cylinder as the root develops, in the latter case in the cortex. In 

 either case the anterior end of the nematode is usually in close con- 

 nection with the cells surrounding the conductive tissues. In the 

 case of larvae which hatch from eggs produced within the root, some 

 bore their way out into the surrounding soil and enter new roots, as 

 described above, while others burrow along in the tissues of the root 

 and settle down, usually in the fleshy cortex. Thus an old nematode 

 gall will contain nematodes in all stages of development and at a 



' Davaine, 1857. Miinter, 1866. Needham, 1745, 1775. Baker, 1753. 

 2 Ritzema Bos, 1892. 

 » Dorsett, 1899. 



217 



