PECAN" EOSETTE. 11 



In varieties where the size and distribution of the yellow spots 

 made it possible, Baur found that by carefully cutting out all yellow 

 spots and continuing this process on all newly developing leaves for 

 one or two weeks, finally green leaves only were formed. From 

 this result he considered it certain that the contagium is present in 

 the yellow spots but only in sufficient quantity to infect about three 

 or four newly developing leaves at the growing point. After this it 

 is apparently used up, and leaves subsequently formed remain green. 

 Darkening the assimilating leaves of a mottled plant led to a similar 

 result. Here the first leaves to develop thereafter were yellow 

 spotted, but if these also were darkened before they began to assimi- 

 late, the subsequently developed leaves were all green. 



Girdling experiments demonstrated that the contagium is carried 

 only through the bark. Different species and varieties of Malvaceae 

 were found to vary widely in susceptibility and also in the incubation 

 period, but several days at least are required. Baur also demon- 

 strated infectious chloroses in Fraxinu^, Laburnum, Sorbus, Ptelea, 

 Euonymus, and Ligustrum. 



In potato mosaic the mottling is irregularly distributed irrespec- 

 tive of the venation ; and, moreover, profound dwarfing, with curling, 

 crinkling, and further distortions of the foliage occur in the more 

 severe attacks of the disease. The parenchyma tissues are less per- 

 fectly developed in the light-colored areas, the palisade cells tiend to 

 shorten up and chlorophyll development is restricted. Potato mosaic 

 (65, 67) is transmitted by the tubers, by gTafting, and by inoculation 

 with the expressed juices of diseased plants; and it is also dissemi- 

 nated by aphids. 



In potato leaf-roll the leaves become inrolled from the margin, 

 reduced in size, and of a paler green to yellowish cast. A necrosis 

 of the phloem (60-62) seems to be characteristic of the disease, but 

 whether this condition is specific for leaf -roll is still a moot point. 

 The disease is transmitted by means of the tubers, by grafting, and by 

 insects (66). 



In both potato mosaic and leaf-roll portions of the healthy plants 

 growing near diseased plants contract the disease, but not all tubers 

 from such secondarily infected plants necessarily develop diseased 

 progeny. Affected leaves in both these diseases exhibit deep-seated 

 assimilatory derangements, including increase in activity of the oxi- 

 dizing enzyms and gorging of the leaves with starch, with greatly 

 delayed starch translocation. In both these diseases tuber forma- 

 tion is greatly restricted. In neither case are these diseases induced 

 or cured by soil or climatic conditions. However, certain environ- 

 mental factors may temporarily mask the external signs though such 

 diseased plants still retain their power to infect. 



