HETERODERA RADICICOLA. 411 



and were thus able to be constantly observed. Galls containing 

 the gravid females commenced to appear on the roots, and wilting 

 set in thirty days after planting the seedlings. 



Mode of Infection by the Parasite and its Effect on 

 the Host Plant. 



The free living larva inhabiting the soil is the infective 

 agent. It penetrates the young roots of the host, especially :n 

 the region of the tender-growing points, and aided by its spear, 

 pierces its way into the vascular bundles and the surroundi?ig 

 tissues. The further development of the larva now takes place, 

 leading, as has been shown to the fertilisation and swelling of 

 the females until they attain the cyst-like form. The gravid 

 females are usually found in groups and seldom occur sol'tavily. 

 In the potato tubers they generally occupy an area about half 

 a centimetre from the edge ; in badly infested tomato and fruit 

 trees roots they are found in large numbers even nearer the 

 periphery. 



The presence of the parasites in the roots acts as an irritant 

 on the plant and leads to a rapid multiplication of the cells of 

 the parenchymatous tissue. When the tissues of the vascular 

 bundles are affected, the vessels grow around the parasites and 

 consequently contortions of the course of the vessels are pro- 

 duced. The abnormal multiplication of the cells result in the 

 formation of "galls" or "knots" which may be up to an inch 

 in diameter and give the roots a much disfigured appearance and 

 interfere with the circulation of the sap. 



An attempt is sometimes made by the host plants to resist 

 the future ravages of the parasite. The walls of the cavities in 

 which the females are situated become brown, and, to a cei'tam 

 extent, bonified, thus tending: to cut the parasites off from the 

 adjacent tissues and to hinder any further migration on their 

 part. 



The interruption of the flow of sap due to the ravages of 

 the parasite results in most cases, where the infection is heavy, 

 in the stunting of the growth of the plant, the leaves often become 

 wrinkled, gradually commencing to wilt and finally dropping 

 off. It seems that it is not so much the actual diversion of the 

 sap and plant juices which act as food for the parasites — for that is 

 comparatively small — that is so fatal to the plant, as the physical 

 irritation resulting from its presence which leads to the derange- 

 ment of the vital functions of the roots. Further, the entrance 

 of the larvae into the roots often allows the ingress of pathogenic 

 bacteria, such as Bacterium solanacearum into tobacco and 

 tomato roots, and the organisms causing the cotton disease known 

 as "Blackroot" in Georgia, U.S.A., which do much to accelerate 

 the fate of the host plant. Badly infested roots are cften soft 

 and pulpy at the site of infection. This may be due to the 

 secretion of a toxic substance by the parasite which affects the 

 tissues and facilitates the escape of the larvae into the soil. 

 On the other hand, the decay of the root may also be caused by 

 bacteria and fungi that are usually present. 



