[Vol. 10 



192 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



plant disease involving chlorosis, whether with or without mot- 

 tling, has not been positively determined, but, for the most part, 

 there are some common characteristics of all to which at least 

 passing reference must be made in the course of this paper. 

 The relation of mosaic diseases to infectious or non-infectious 



a 



and green) of ft 



remains for detailed study. In this paper we propose to discuss 

 more particularly some of the problems relating to infectious 

 chlorosis with special emphasis on that type illustrated by the 

 mosaic disease of tobacco. 



The true mosaic " diseases" of dicotyledons constitute a some- 

 what homogeneous group, for they exhibit a blotching or mottling 

 efined as a mosaic — in which usually both a hypoplastic and 

 a hyper] >Iastic development of the tissues ensues. The mottling 

 is very largely confined to the leaves and may be characterized 

 by regions of lesser chlorophyll development (often a definite 

 yellowing or chlorosis) and regions of intensified chlorophyll 

 development. The latter has sometimes been treated as a quanti- 

 tative intensification of the chlorophyll and the former as a 

 degradation or diminution of chlorophyll. In discussing the 

 symptomatology of such diseases it has been more the custom 

 to emphasize as the disease the chlorotic areas; and certainly if 

 the sugar cane, maize, and certain other monocotyledonous 

 " mosaics" are included in the category of true mosaics, then the 

 chlorotic areas are admittedly strongly diseased. Abnormal 

 greenness may be, nevertheless, so characteristic that this too 

 should have its place among the symptoms. 



Among those who have studied tobacco, bean, and similar 

 mosaics the view has been held also that the intensified green 

 areas are primarily those of disease. In the studies of Dickson 



both 



of chlorophyll change. Here, as in the earlier ini 

 of Iwanowski ('03), it is clearlv shown that in the 



•eenness is correlated with, and in part (at least] 

 d development of chlorophyllous tissues, — hyp 



change: 



which is produced 



differentiation 

 : formed after 



infection. There is no such result in organs alreadv mature 



