34 BULLETIN 1038, U. S. DEPARTMENT OF AGRICULTURE. 
and chemical reactions and stimuli, and in their application in a 
particular manner and at a particular time and place, that the com- 
plexity of the final manifestations of infectious diseases differs from 
the more simply induced changes in metabolism and structure. It- 
is this regulatory effect and this extreme complexity of reaction 
which make many infectious diseases so difficult to induce by arti- 
ficial means. 
If causes may in any measure be judged from their effects, the 
histological and cytological evidence points to the cause of pecan 
rosette as being similar in its general nature to the ultimate causes 
of the infectious chloroses. Whether in this particular disease the 
factors responsible for alterations in the normal structure and metab- 
olism must be introduced into the plant from without, or whether 
they originate within the plant itself, is a question yet to be answered ;, 
but whatever the ultimate solution of the problem may be, the cause 
will undoubtedly not be found in any simple soil or water relation. 
So far as worked out, the infectious chloroses in general exhibit, 
like pecan rosette, a simultaneous and deep-seated derangement both 
in morphogenesis and metabolic processes. Structural derangements 
in diseased leaves may show abrupt local change. That is, in many 
cases, entirely different types of tissue derangement, such as reduc- 
tion in size of cells and number of cell divisions or enlargement of 
cells and increased number of cell divisions, may occur side by side, 
not only in the same leaf but even in the same part of the leaf. 
Along with these structural changes occur fundamental functional 
derangements concerned with the assimilation and translocation of 
carbohydrates and with nitrogen metabolism. As is well stated by 
True and Hawkins (79) in considering spinach blight: 
It would seem to be indicated that the cause of carbohydrate accumulation 
should be sought in the deeper lying metabolic processes in connection with 
which carbohydrates are utilized. . . . Accumulation is due not to a breakdown 
of digestion but to some partial failure in the subsequent metabolic processes 
in connection with which carbohydrates are used. 
Moreover, not only restriction in chlorophyll development occurs, 
but often abnormal and irregular groupings and a reduction in size 
and number of chloroplasts. In the later stages disintegration of 
the plastids and other cell contents often follows, and finally a 
shriveling of the entire cell ensues. 
It is typical of the group of infectious chloroses that only meriste- 
matic tissues are morphogenetically affected. There is always a 
rather definite incubation period for each disease, and external signs 
of the disease gradually progress from the point of entry of the 
contagium. In some instances the contagium appears not to reach 
all parts of the plant, as in the case of potato plants secondarily 
infected with mosaic or leaf-roll where often only a part of the 
