20 Graft- Infectious Disease of Datura 



III. Origin of Quercinas in the Field. 



The description which we have given of Q. plants is based on in- 

 dividuals which had come from Q. seed. The condition, however, 

 breaks out in the field, generally late in the season on plants which 

 have been produced by normal parents. The infection may be first 

 noticed weakly expressed in a single branch, and gradually spreads as 

 new growth occurs. Fig. 6 is a photograph of such a branch which 

 has a normal spiny capsule at the lowest fork shown. The later 

 capsules show a gi-adual suppression of spines to the one which is 

 entirely smooth. Occasionally an infected capsule is found with one or 

 more of the four valves smooth and the rest spiny as shown in Fig. 7 

 (insert, in Fig. 6). The two flowers of the infected branch, above and at 

 the right, have the characteristic separation of the corolla into segments. 



The presence of the disease may be first observed in the change in 

 form of leaves and flowers. Purple flowers become darker in colour 

 and often slightly mottled with lighter patches, before the infection 

 has brought about a splitting of the corolla. As the season advances 

 more plants show Q. branches. When normal plants have flowered 

 and formed capsules, Q. plants are still flowering; for the Jimson 

 Weed is normally self-pollinated in the bud, and Q. plants, from their 

 lack of pollen, are dependent for setting capsules upon occasional off- 

 pollination which is especially rare in such plants on account of the 

 failure of many flowers to open. 



Table I shows the number of Q. plants found in normal and Q. 

 pedigrees in the field for the year 1916. In 1917 in the same field there 

 were 1'67 °/^ spontaneous occurrences of Q.'s out of 5230 individuals 

 planted in the ordinary manner. No Q.'s from seed were planted in 

 the plot this year. 



With only a single possible exception have Q.'s ever been observed 

 to occur spontaneously in the greenhouse, where Q. plants as well as 

 normals were under cultivation. Under greenhouse conditions plants 

 are not grown as large as in the field. This fact would give a lessened 

 opportunity for infection, but in the many thousand plants infection 

 would have been expected in a few if the means of transmission were 

 present. It is perhaps similarly because of the lack of means of trans- 

 mission that Q. infections have been rare in our plots near the green- 

 houses, while a mile away in our hill plots they have been common. 

 Thus, while in 1917 there were 1*7 "/„ in the 5230 plants in the regular 

 hill plots, there were no infections in over 2000 plants in the plots 



