10 BULLETIN 842, U. S. DEPARTMENT OF AGRICULTURE. 



have grown together and usually show separate interior cavities, as 

 well as deep external furrows, along the lines where the separate 

 units have coalesced. The simple galls are by far the more common 

 and usually occur singly in each flower, although as many as three 

 separate galls have been found in a single flower. Sometimes a 

 3 T oung gall may be seen developing in place of one stamen, while the 

 other two stamens and the pistil still appear almost normal. It is 

 rather rare, however, for a normal kernel and one or more galls to 

 develop within the same flower. A young gall is not uncommonly 

 found in place of each of the three .stamens, and later these may 

 produce a single trilobed compound gall. In such cases ovarial de- 

 velopments are absent. More commonly, however, a single gall is 

 initiated either in the ovary or the young staminate tissues, and it 

 then usually causes atrophy or nondevelopment in the other repro- 

 ductive parts of the flower. 



Only the reproductive organs or adjacent tissues of the flowers 

 have been found affected, but every flower on a spike may be in- 

 vaded. It is not uncommon, therefore, to find every flower of a 

 mature spike containing nothing but galls. Heads of wheat well in- 

 fected with larvse are often reduced in size, have their glumes stick- 

 ing out at a greater angle from the rachilla than normal ones, and 

 mature somewhat later than the uninfected spikes. Because of their 

 thickness, the dark maturing galls are often not entirely covered by 

 the glumes, so that they become exposed and thus serve as a con- 

 spicuous symptom whereby infected spikes may be readily distin- 

 guished in the field (PL III). 



CAUSE OF THE DISEASE. 



The cause of the nematode disease discussed in this paper may, 

 be readily seen with the unaided eye when the contents of a gall 

 are placed in clear water. With the aid of a hand lens, the milky 

 white fibrous mass thus liberated, as shown in Plate V, B, can be 

 readily observed to consist of thousands of straight or curved 

 threadlike elements, the larvse of the nematode Tylenchus tritici 

 (Steinbuch) Bastian. If watched carefully they will be seen soon to 

 begin active eellike movements. Occasionally among a white mass 

 of larvse from a mature gall one may see with the naked eye the 

 brown misshapen remains of adult males and females, and with the 

 assistance of a microscope find a few eggs, the stage from which 

 the larvse developed. In young galls, living adults of both sexes 

 and an abundance of eggs and larvse may be found. 



EGGS. 



When viewed from the end the eggs seem to be almost perfectly 

 circular in cross section. In lateral view, which is that most com- 

 monly seen (fig. 2), they are elongate, granulated bodies, usually 



