PECAN ROSETTE. 35 



tubers give rise to diseased progeny. Furthermore, the fact that 

 many of these diseases are transmissible by insects shows them to be 

 entirely different in nature from those chloroses due to soil or cli- 

 matic conditions. In some of these diseases where the insect rela- 

 tions have been most carefully v/orked out it is indicated that a 

 definite incubation period must also elapse after feeding upon a 

 diseased plant before an insect becomes infective. 



Except in the most general way, the infectious chloroses exhibit no 

 pattern as related to the leaf. The factors controlling morpho- 

 genesis of the leaf in many cases seem to have run riot. In the 

 mosaic type the spots, irregular in themselves, are also irregularly- 

 distributed over the surface of the leaf. In many cases, it is true,, 

 they avoid the larger veins, but in other instances they are distributed 

 irrespective of venation. 



There is nothing to show a difference in kind between those mosaics, 

 transmissible by expressed plant juices and those transmitted only 

 by grafting, such as the infectious mosaic of Abutilon. The causal 

 contagium in the latter case may be compared to a more obligate 

 parasite which is greatly restricted in the conditions necessary to its 

 life activities and reproduction. 



The fact that later investigation has shown some of the filterable 

 contagium diseases to be due to microorganisms is at least presump- 

 tive evidence in favor of the organism theory of infectious chloroses.. 

 Furthermore, Allard has shown that with a fine enough clay cup it 

 is possible entirely to filter out the infective principle from the 

 expressed juices of tobacco plants affected with mosaic. 



The crucial difference, however, lies in the power of self-reproduc- 

 tion possessed by the contagia of all the infectious chloroses. To 

 give a concrete example, starting with a mosaic-diseased tobacco 

 plant and in each case using a drop of the expressed plant juice di- 

 luted 1 to 1,000 in water, it is possible to cause the disease in an in- 

 definite series of plants successively inoculated one from the other at 

 proper intervals, and the juice from the last of the series will possess 

 as high a power of infection as that from the first. In a case like 

 this it might be mathematically shown that if reproduction had not 

 taken place a drop from the last plant of the series must contain less 

 than one molecule of the originally injected material. No such power 

 of self -reproduction has ever been demonstrated for a definite chemical 

 compound. The true enzyms are formed by living organisms in re- 

 sponse to certain stimuli and have never been shown to be capable 

 of self-reproduction. 



Assuming the contagia of infectious chloroses to be nonliving sub- 

 stances, their effects may be compared up to a certain point with 

 various phenomena of bacterial toxins and immunization. However,, 

 the fact of reproduction in these contagia is in no way elucidated by 



