1923] 



DUGGAR & KARRER 



207 



('18) the inheritance of a number of chlorophyll types is shown 

 to be strictly Mendelian. These types involve various degrees 

 of striping and cases in which the chlorophyll is almost or entirely 

 suppressed, with the production of white, virescent, or yellow 

 seedlings. Passing from these normally inherited color character- 

 istics to those which are infectious, such cases as those of the 

 variegated Abutilon and the striped Ligustrum, worked upon by 

 Baur ('06, '07, '08), come up for consideration. In these cases, 

 it will be recalled, there is a characteristic pattern of color, but 

 there is no noticeable tissue modification. Transmission is by 

 grafting only. 



The types just discussed, without graft-infection experiments 

 to demonstrate their peculiarities, would be considered "nor- 

 mal" variations. The infectiousness, however, is precisely that 

 which was found by Erwin Smith to prevail in peach yellows. 

 "Peach yellows" is, in part, a chlorotic disease, but it gradually 

 leads to severe injury and ultimately death of the peach tree. 

 The disease is not transmitted by pollen nor, so far as known, 

 by seed, but a diseased scion will convey the disease in time to a 

 healthy stock. This disease has been considered by many to 

 possess a highly infectious nature, but of this infectious char- 

 acter the senior writer has been wholly unable to find any 

 authentic proof. Statements indicating that it may "sweep an 

 orchard in a few years, " when followed up are found to be equally 

 as well explained by the possibility that all the stock came from 

 a single nursery at the same time. Scions from the same tree 

 may have been employed. This disease, moreover, is rather 

 closely localized in a narrow climatic zone. The claims of sporadic 

 appearance of the disease in regions far south of Michigan and 

 Delaware have in very few, if any, cases been adequately verified, 

 especially since the water-shoots arising in clusters from severely 

 headed-back or winter-injured trees possess many characteristics 

 of yellows, clearly recognized, however, as water-shoots by the 

 expert. 



One should include in the graft-transmissible forms reference 

 to the recent work of Blakeslee ('21), in which a graft-infectious 

 disease of Datura resembling a vegetative mutation is discussed 

 and its behavior in heredity clearly set forth. This disease has 



