CHAPTER XXVI 

 INTERNAL CAUSES OF DISEASE 



During recent years attention has been called to diseases which are 

 evidently due to the action of an enzyme, or ferment in the plant, 

 which renews itself perhaps as a catalytic agent in the tissues of the 

 host. As it is filterable through a Berkefeld filter, it may be a soluble 

 enzyme pure and simple, or it may be one of the extremely minute 

 ultra-microscopic organisms to which attention has been called recently. 

 All the evidence seems to point to its enzymatic nature. Such diseases 

 are caused by the excessive activity of the oxidase and peroxidase 

 enzymes in the plant and the loss of function of catalase, another en- 

 zyme, which carries ofi some of the residual products of the others 

 mentioned. Such diseases due to a Contagium vivum fluidum, affect a 

 number of plants, notably the tobacco, and all of these diseases seem 

 to be more or less related, as to their nature and origin. Recently 

 Kiister in the second edition of his "Pathological Plant Anatomy" 

 (1916) has grouped many of the enzyme-produced conditions under 

 the head of "Panaschiering." He distinguishes several t)rpes. The 

 first is when the greeh parts contract sharply under the pale parts. 

 Under this head he considers: (o) marginal panaschiering, when such 

 terms as "albo-marginatis" would be applicable, as in such cultivated 

 plants as Pelargonium zonale, Hedera helix and Weigelia rosea, (b) 

 In sectional panaschiering, the white and the green colors are dis- 

 tributed sectionally over leaves and stems, as in Chamaecyparis pisi- 

 fera plumosa argentea. (c) He distinguishes marbled and pulverulent 

 panaschiering. His second group includes cases where the border 

 between green and pale parts is not sharply marked and this group 

 includes (a) Zebra-panaschiering, as in the banded leaves of Eulalia, 

 and (b) flecked panaschiering, where white specks are distributed over 

 a green background and blend with it. It is clear that "Mosaic," 

 "Brindle," "Calico" or "Mottle Top" of tobacco is a physiologic, 

 not a fungous or bacterial disease. 



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