22 G^^aft- Infectious Disease of Datura 



Since the typical Q.'s are devoid of pollen it has been necessary to 

 cross them with normals. Table I (p. 32) shows the results of such crosses 

 from field records of 1916. Q.'s are produced in 79 7o of the offspring 

 of Q, female parents. The remaining normal plants from Q. parents do 

 not carry the infection. The incidence of spontaneous infection in the 

 field was even somewhat less for them (0"95 °/^) than for normals free 

 from Q. ancestry (1'26 °l ^. 



Q. plants from Q. parents usually show their Q. nature at an early 

 seedling stage. In 1916 the seedlings from Q. parents were labelled 

 "Normal," "Questionable," or "Quercina" at the time of setting into 

 the field from 3-inch pots. When the adult plants were inspected in 

 the fall, their condition was compared with the earlier records on the 

 pegs that remained. The results are tabulated in Table II (p. 32). 



Plants classified as Q.'s in seedling stage are Q.'s when adult. The 

 tw^o exceptions noted out of 671 plants, if not the result of experi- 

 mental error, may be due to faulty classification in the earlier record. 

 It will be noted that in seedlings the Q. infection is determined by 

 leaf abnormalities alone. Apart from these two possible exceptions we 

 have not observed that the plants once infected ever recover. The 

 infection often manifests itself, however, only after the plants have 

 been transplanted to the field, as is shown by the 17 ""/^ of apparently 

 normal seedlings which later became Q.'s. There is usually little 

 difficulty in distinguishing Q. plants out of infected seeds from field 

 infected individuals, in which the character is evident later in the season 

 and does not involve the older leaves. About as many " Questionable " 

 seedlings turned out normals as there were normal seedlings which 

 turned out Q. If all the questionables were included with the Q. 

 seedlings they would have given in consequence a close approximation 

 to the total Q.'s found in the adult stage. 



Some of the plants put into the field died before final records were 

 taken, and some of the labels were lost. The table includes only 

 individuals with complete records. Of 77 plants not included in the 

 table which died in the field, 24 were described on the pot labels as 

 Q.'s, 51 as Questionables, and two only as normals. In other words 

 3*4% of the seedlings classified as Q.'s died in the field, 12*8 7o of 

 those classified as Questionables, and only 0*7 °/^ of those classified as 

 normals. The questionables were probably thus classified on account 

 of not having developed far enough to allow a sure identification of 

 their character. They were thus weaker plants when set out, hence 

 their greater mortality. If we class all the Questionables with the 



