NEMATODE DISEASE OP WHEAT. 15 



them. The writer was unable to produce root infection by placing 

 large numbers of active larvae on the surface of sterile tubes of a 

 plant nutrient agar containing wheat seedlings. In such cultures 

 the organisms were observed to move about freely through the trans- 

 parent medium and to accumulate about the seedling roots, mostly 

 near the root tips, as shown in Plate V, A, but no penetration was ob- 

 served. There was not, however, the marked accumulation about the 

 root tips which the writer (5) has previously observed when pure cul- 

 tures of the root-knot nematode (Heterodera radicicola) were inocu- 

 lated in similar tubes containing tomato plants. In the latter case 

 there seemed to be some sort of stimulus, possibly chemotactic, attract- 

 ing the larvae to the growing point of the root, which many of them 

 entered. Many examinations of wheat seedlings growing in the 

 greenhouse in soil well infested with larvae of Tylenchus tritici 

 showed no root infection. 



After having reached the aerial portions of their young host plants 

 some of the wheat nematode larvae may enter the leaves, but most of 

 them locate in large numbers at the base of and between the leaf 

 sheaths near the terminal growing bud. Here they remain in an 

 ectoparasitic condition, ready to attack the wheat heads as they 

 develop. Thus, contrary to what is commonly thought, the organism 

 of itself travels only a short distance, possibly less than an inch 

 sometimes, in reaching the wheat spikes. The exact manner by 

 which larvae enter the leaf or flower tissues has never been observed. 

 As there appear to be no natural openings, such as stomata, large 

 enough to permit their entrance, it must be assumed that they get 

 into the inner tissues either by artificial openings or, as is much more 

 likely, by a piercing of the cells with their spearlike anterior ends. 

 At the same time they may possibly secrete some substance to assist 

 in breaking down the cell walls. Only the young, actively growing 

 tissues and cells are invaded. 



Wherever larvae penetrate the leaves, an increase in the size and 

 number of host cells takes place, resulting in a local gall. One or 

 more of these galls may be formed on a single leaf, and they may be 

 located on any portion of the latter, depending upon the place and 

 time of larval penetration. The larvae usually enter only the young 

 leaves which are entirely inclosed within the** older leaf she'aths. 

 Their entrance somewhat retards and prevents normal development, 

 causing the leaves to grow unevenly and to become wrinkled, rolled, 

 and distorted. These symptoms are illustrated in Plate II, A and i?, 

 which in addition shows a small gall on the edge of the youngest leaf. 

 This gall, which is typical, is about 4 mm. long, 3 mm. wide by 

 2 mm. thick, and has lost its normal green color, being almost white. 

 Upon teasing it apart 25 of the nematodes were found in various 



