1923] 



DUGGAR & KARRER — NATURE OF MOSAIC DISEASE PARTICLES 193 



the time of infection. Woods ('00, '02), Chapman ('17), and 

 others have also given attention to the anatomy of mottled areas. 

 The yellow or chlorotic areas are ordinarily correlated with regions 

 of lesser development of the chlorophyllous tissues, — with hypo- 

 plastic development. It is of some interest to note that in a 

 typical mosaic disease of swiss chard observed by the senior 

 writer at the Missouri Botanical Garden in 1919 the chief, if not 

 sole, visible color effect was intensified greening in a blotched 

 pattern. In general, there is in dicotyledonous mosaics a focal 

 distribution of effects, and possibly it may be strictly analogous 

 to conditions in human measles, or to the effects produced by 

 an injection into the body of diphtheria antitoxin, or to the focal 

 distribution of pigment in certain skin diseases. In the mosaic 

 disease of tobacco, necrosis, as generally understood in plant 

 pathology, does not occur. 



It should not, however, be assumed that marked mottling is 

 necessarily a symptom of these diseases, since many plants or 

 plant species, not themselves seriously mottled, may exhibit, 

 through infection experiments, evidence of severe attacks of the 

 disease so far as this may be expressed through the infectiousness 

 of their juices. Moreover, dwarfing of the general plant, spindling 

 shoots, abscission of blossoms, and many other characteristics 

 are typical of mosaic diseases as they are understood in certain 

 plants, notably in the potato and in certain cucurbits. 



Mosaic diseases are recognized to occur in many species of 

 Solanaceae (night-shade family) , Cucurbitaceae (gourds, squashes, 

 etc.), Leguminoseae (peas, clovers, etc.), C henopodiaceae (beet 

 and spinach family), Rosaceae (raspberries, etc.), Gramineae 

 (grasses, sugar-cane, etc.), and many others, altogether about 



20 families. 



There have been several possible views as to the nature of the 

 causal agency or agencies in mosaic diseases, all or nearly all of 

 which have been exploited, and perhaps very nearly discarded. 



ENZYME THEORY 



The chief adherent of the enzymic nature of mosaic disease 

 has been Woods ('99, '00, '02). He postulated that the cause 

 of mosaic may be found in an enzyme disturbance in which the 



