DISEASES OF CULTIVATED PLANTS 443 



ease in tobacco, and in general variegated or special yellow foliage 

 types of plants as in Arundo, Acer and other genera of plants belong 

 here. The yellows in peach has long been studied, as also the tobacco 

 mosaic disease. In yellows the contagious character of the disease 

 and its transmission in pruning by contact of parts of the harness of 

 team and by or through the atmosphere has been recorded. 



A few years ago it was determined by Beierjink and by Hunger 

 that this infection exists as a chemical compound or compounds of 

 complex nature belonging to the oxidizing ferments of a group called 

 the oxidases. Oxidase, peroxidase and others of these ferments are 

 known. They act by breaking down or oxidizing the plant leaf tis- 

 sues and especially the chlorophyll or leaf -green of foliage and young 

 tissues, converting it into xanthophyll. The tests for these ferments 

 are of some importance. Woods and others have shown their action 

 with peroxid of hydrogen. 



From a practical point of view the transmission of the ferments, 

 and, therefore, of the disease, by touching first diseased and then 

 healthy foliage is rather surprising, The work of Hunger in Java 

 upon the transmission of the tobacco mosaic disease makes the risk 

 of transmission from diseased to healthy plants by such handling, 

 stand out clearly. This line of transmission was verified on tobacco 

 by the writer's assistant in 1903. (See Bulletin 156, of the Ohio 

 Station.) 



While the same class of proof for peach yellows is very difficult, 

 owing to the latent nature of the disease for some months after first 

 infection, the actual results of infection from nearby diseased trees 

 make clear the danger of such exposure and the necessity for the 

 destruction of diseased trees. Chemical examination of variegated 

 or chlorose tissues shows the same compounds, the oxidases, etc., to 

 be present and to account for the transformation of the leaf-green 

 or chlorophyll, into xanthophyll, or leaf yellow. Thus by degrees 

 apparent plant disease mysteries are solved. The weakness of vari- 

 egated plants and their ready susceptibility to attacks of parasitic 

 fungi are now explained by this impaired condition of the leaf parts. 



PLANT DISEASES TRANSMITTED IN THE SEED. 



The public in general little realizes how many diseases of plants 

 are transmitted in the seed, although as the years pass the general 

 dissemination of knowledge concerning infection by spores and by 

 germs has partly prepared the way. The public mind does not longer 

 expect something to grow from nothing. The treatment of seed 

 grain, as wheat, oats, barley, etc., to destroy adhering spores of the 

 smut fungi, and thus prevent these smuts in the crop, has been 

 known for many years. In the early days of the Agrk-ultural Ex- 

 periment Stations, these doctrines and practices in this regard were 

 widely disseminated, new impetus being given by the successful use 

 of hot water following the methods of Jensen in Denmark; but 

 despite the conquest of the practice control over the order Ustiligi- 

 neae, the smuts, we have only really begun to study the matter of 

 seed infecting diseases produced by seed infesting fungi. These 

 geed infesting fungi are of two types, viz., first, those whose spores 



