PECAN EOSETTE. 33 
It is difficult to explain on the soil hypothesis why only a part of 
a tree may be diseased and why when two trees of the same age stand 
within a few inches or a few feet of each other the one may remain 
perfectly normal and vigorous while the other is stunted and dying 
back with rosette. The recovery of diseased trees when transplanted 
in the north on the other hand is also difficult to explain, but no 
more so than the apparent recovery of potatoes from mosaic when 
carried from Maine to Colorado or tobacco mosaic under certain rela- 
tions of light or temperature. As previously mentioned, Baur has 
clearly demonstrated that the contagium of abutilon mosaic is readily 
killed by subjection to certain environmental conditions such as the 
withdrawal of light. He found that cutting out the yellow areas 
as they developed also finally brought recovery. Here, as in many 
parasitic diseases both of plants and of animals, though the conta- 
gium may be carried to remote parts of the body, it is only in certain 
definite locations and under certain definite conditions that it can 
reproduce itself and initiate lesions in the host. 
If pecan rosette is due to some chemical compound brought in from 
the soil in harmful quantities one would expect the tissues along the 
veins to be first and most profoundly affected and that the result 
would be evident over the whole tree at about the same time. Fur- 
thermore, if the yellow mottling is interpreted as due not to a cause 
operating from the focal centers of the spots but to a lack of suffi- 
cient soil nutrients or water brought in by the veins, how are to be 
explained the lack of chlorosis along the leaf margins and the abnor- 
mally increased growth at the periphery of the spots ? 
It is true that the local application of purely physical or chemical 
stimuli may locally cause cells to enlarge or proliferate, and iheir 
application in lethal quantities may result in injury and final death 
without the intervention of any parasitic organism, as witness, for 
example, the more recent experiments of Dr. Erwin F. Smith (74) 
in the production of plant overgrowths without the intervention of 
any parasite. However, as in any cytological or embryological study, 
it is not the single section taken by itself that tells the story, "but the 
sequence of one following the other, the whole series fitting together 
in an orderly manner to build up the complete picture, so in the 
chloroses of plants it is not the mere fact of chlorophyll disintegra- 
tion that will show the type of disturbance present, or that will 
eventually lead to the determination of the cause in any particular 
case, but the whole series of events and appearances concerned in the 
production and manifestation of the derangement. 
It is probable that all effects of parasites upon their hosts, when 
reduced to their ultimate reactions, may be explained in terms of 
physics and chemistry. It is in the regulation of these physical 
