Plants in their Relation to Disease. 17a 



During the period of active vegetation, the leaves and all other green parts^ 

 of the plant assimilate carbon dioxide and water for the formation of starch, 

 which is not only transported to other parts to meet the immediate require- 

 ments of growth, but a certain portion is deposited in the assimilating tissues- 

 themselves. Towards the close of the season, as the functions of the leaves 

 gradually cease, no new starch is produced, and that already present finally 

 undergoes complete transformation into oil. At the same time the color of 

 the organ changes by a process of degradation in the chlorophyl. We thus- 

 have annually repeated both fatty degeneration and degradation of the 

 chlorophyl pigment, as indicative of and incident to the normal animal 

 maturity of parts. When from unusual conditions, however, these changes 

 are caused to appear at any other than their normal period, they become 

 sure indications of premature development through disordered function; so 

 that when a peach leaf assumes a bright-yellow color in June, when it should 

 be deepest green, we may feel certain, as is actually the case, that all its 

 amylaceous cell contents have undergone fatty degeneration, and that it is 

 an indication of disease which must be heeded. Similar changes are asso- 

 ciated with pear blight, and doubtless with other diseases. The experimental 

 evidence obtained by Dr. Cunningham,* not only demonstrates that such 

 changes may be produced artificially by controlled starvation in both plant 

 and animal, but they serve as a most interesting confirmation of the results 

 obtained in a similar direction with reference to peach-yellows. 



Atrophoid structures are always indicative of disease. They have been 

 observed to occur in the experiments of Mobbe and Shroeder with buckwheat ; 

 they are also known to occur in peach-yellows, of which they constitute one 

 of the characteristic symptoms of advanced development. Such depaujjerate 

 growths are known to originate in imperfect nutrition of the growing parts, 

 so that whenever present they give us a certain clew as to the general course 

 of treatment to be followed. 



From these considerations it is obvious that we can not hope to reach any 

 correct diagnosis upon the basis of one or two external symptoms alone ; we 

 must in general go much beyond that, and have due regard for those which 

 are internal as well. We now have to consider by what means a disease 

 which originates locally may finally involve the entire system ; by what gen- 

 eral process it may be distributed through the organism. 



In the case of fungoid i>arasites, it is not difficult to see that, with the lapid 

 extension of the mycelial filaments through the organism and the very 

 abundant development of spores, not only are conditions developed in the 

 plant which favor the accelerated action of the parasite, but if the latter pos- 

 sess pathogenic power, disease arises constantly in new centers, and rapidlj'- 

 spreads through the whole structure. Owing largely to the physical obsta- 

 cles opposed to the extension of the parasite, this, too, frequently fails to sat- 

 isfactorily account for the very rapid diffusion of disorder throughout the- 

 system. 



^Quarterly Journal Microscopical Science, January, 1880. 



