Chap. 26 ROUNDWORMS THE TUBULAR PLAN 523 



species of plants while the closely related common garden roundworm, Melo- 

 idogyne marioni, inhabits plants of over 1000 varieties. The worms lay their eggs 

 either in the roots or in nearby soil. In either case, the young larvae bore their 

 way into the rootlets. The plant cells are stimulated by the foreign body and 

 divide rapidly, forming little galls, or root-knots, in which the parasite is walled 

 in by scar-tissue (Fig. 26.3). The roots soon become so deformed that they 

 cannot function and the plant dies. In both plants and animals, the tissues of 

 the hosts often develop growths or secrete substances that wall in the parasite. 

 Nematodes also enter leaves, usually through the breathing pores (stomata), 

 and move about the latticed interior, eating out the contents of the cells as they 

 go (Fig. 26.1). On the outside, the disturbance is marked by twists in the 

 leaves and by whitened trails. Nematode parasites are harbored by water as 

 well as land plants. Even sea weeds (Ascophyllum of the Atlantic coast) may 

 be burdened with nematode galls. 



Fig. 26.3. Knot-root caused by a microscopic nematode. Meloidogyne marioni. 

 Knot-root galls cause great loss to vegetables especially cabbage and its kin, 

 cotton, and several of the grains: A, tomato; B and C, parsnips. Every knot-root 

 gall is populated by millions of nematodes. (After Jeffers and Cox. Courtesy, 

 Walker: Diseases of Vegetable Crops. New York, McGraw-Hill Book Co., 1952.) 



