DISEASES OF PLANTS, 



395 



BRANCHES. 



Healthy 

 (restored). 



Diseased. 



Ferric oxide . . . . , 

 Calcium oxide . . . 

 Ma^esium oxide . 

 Phosphoric acid . , 

 Potassium oxide . 



0-52 

 54-52 



7-58 

 11-37 

 26-01 



1-45 

 64-23 

 10-28 



8-37 

 15-67 



Total 



100-00 



100-00 



It has further been shown by myself * that in some cases of disease, 

 notably the one just referred to, there are important modifications of 

 cellular structure and cell-contents as typical of the pathological con- 

 dition. The growth becomes depauperate as a whole, and also in the 

 various anatomical elements of the structure. At the same time the 

 foliage assumes an abnormal color, and fails to perform its functions in 

 the assimilative process. Yet, again, with reference to the storage of 

 assimilated material, there is often an abnormal accumulation of such 

 elaborated food in parts where it should not appear, except in limited 

 quantity. 



All these conditions in the case of peach-yellows are coincident 

 vnth. the development of the peculiar chemical conditions as noted 

 above ; and it is further a most interesting fact that, while the excess 

 of lime and want of potash occur together with depauperate struct- 

 ure, loss of color in the foliage, and excessive storage of starch, an 

 increase of the potash and decrease of lime occur simultaneously 

 with a disappearance of these various abnormal conditions. 



We are now led to inquire as to the proper course to pursue in mak- 

 ing a diagnosis. 



When the disease is strictly localized, as when produced by inju- 

 ries, or by the action of parasites proper; when the local disturbance is 

 of sufficiently recent origin to render it improbable that the general 

 system has become involved — then the diagnosis is in most cases a 

 simple matter, and chiefly involves the correct recognition of the 

 cause of the disturbance — ^i. e., the name and character of the insect or 

 parasite, or the particular means by which the injury was first in- 

 flicted. 



When the disease involves the entire system, and the conditions 

 become more complicated, then the difficulty increases. A correct and 

 complete diagnosis can then be made only when we consider — 



1. The chemical composition in health and disease. 



2. The internal features, including the — 



(a) Cellular structure. 



{h) Cell-contents. 



(c) Presence of fungi in the cells. 



3. The external features, embracing the — 



(a) Color and size of the foliage. 

 * " Transactions of the Massachusetts Horticultural Society," 1882. 



