PECAN KOSETTE. 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 recoverj^ 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 

 exj)lained 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 their 

 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 



