18 Graft- Infectious Disease of Datura 



on jjeas, Naudin (7) and Godron (6) had used this species in studies of 

 inheritance and with it had discovered the reappearance of the con- 

 trasting parental types in the i^a generation. De Vries (8) and later 

 Bateson and Saunders (2) used the same species soon after the re- 

 discovery of Mendel's Law in studies on the inheritance of spininess 

 and colour. The present writer with Avery (4 and 5) has also accumu- 

 lated additional data on the inheritance of these and other characters. 



Plants showing the disease have been called Quercina (abbreviated 

 to Q.) on account of peculiarities in leaf structure. The first Quercina 

 plant was found in cultures at the Conn. Agric. College in the fall of 

 1915. It was considered a spineless mutant of the purple-stemmed 

 form. Two open-pollinated capsules had set, and two flowers later 

 opened, which, since their stamens were devoid of pollen, were arti- 

 ficially pollinated by other plants. The seed thus obtained was the 

 origin of a series of breeding experiments with the Quercina mutant as 

 it was then called. 



II. Description of Quercina Plants. 



In describing the distinguishing characteristics of Quercina plants 

 it will be convenient to start with seedlings from the open pollinated 

 capsules obtained in 1915. As will be shown under breeding experi- 

 ments, Quercina plants do not breed true, but throw a small proportion 

 of normals along with Quercina seedlings. The latter usually can be 

 recognized at an early stage, sometimes as soon as the cotyledons have 

 expanded. PI. II, fig. 1 shows above three Q. plants in 3-inch pots in 

 contrast with three below from the same pedigree which were considered 

 normal at the time the photograph was taken. The early leaves of Q.'s 

 tend to be narrower than normals, more or less twisted, and with 

 indentations at the margin. PI. II, fig. 2 shows the same plants at a later 

 date but similarly arranged. The increasing indentation of the leaves 

 in comparison with the normals is evident. The plant at the lower left- 

 hand corner which previously was classed as normal has now shown its 

 Q. nature by the character of its later leaves. At times, Q. seedlings 

 appear normal till a relatively late stage, but in general a seedling 

 which has the character shows it early. 



A series of Q. leaves are shown on PL IV, fig. 5 below in contrast 

 with a series of normal leaves above from the same line. The marked 

 indentation of the leaves shown in the photograph is not an extreme 

 condition. The leaves on some Q.'s may be eroded so far as to leave 



