4i8 SCIENCE PROGRESS 



definite pathological condition of that part ot the vascular 

 strands through which the elaborated food is conveyed to the 

 growing or storage parts. The cell walls of the phloem swell 

 and turn yellow and are altered in their chemical composition. 

 Associated with this is an impairment of the function of the 

 phloem characterised by failure to convey the normal food 

 constituents from the leaves in which they are formed to the 

 growing tissues in which they are required. The leaves remain 

 choked with starch when those of normally functioning plants 

 growing alongside are empty. Phloem-necrosis has actually 

 been detected in only a few cases : in leaf roll of potato, sereh 

 disease of sugar-cane, mulberry'- dwarf in Japan, mosaic disease 

 of the sugar beet, the sieve tube disease of coffee in Surinam, 

 and the brown bast disease of rubber. In certain other cases, 

 however, particularly in peach yellows and the allied spike 

 disease of sandal, there is such a marked interference with the 

 translocation of starch from the leaves that there would seem 

 to be some functional disturbance in the phloem, though 

 hitherto no necrotic changes have been described. 



Taking these four groups, and a few cases in which it is as 

 yet uncertain whether the diseased condition is transmissible to 

 healthy plants and if so in what manner, there are altogether 

 some seventy or eighty genera of plants belonging to about 

 thirty families which are affected by one or more of these 

 diseases, and the number is rapidly being added to. As know- 

 ledge increases, it is probable that the rough classification here 

 given will have to be modified : in particular the peach yellows 

 type may be merged in the phloem-necrosis group. 



Methods of Transmission 



All of the cases that have been tested so far are transmitted 

 to healthy plants by budding, grafting, or other organic union 

 between diseased and healthy plants. Transmission by mere 

 contact of uninjured surfaces has not yet been detected in any 

 case. 



The next most general method of transmission is by means of 

 sucking insects, generally green-fly (aphids). This has not been 

 established in the case of the infectious chlorosis group, and 

 there appears to be no evidence that this type is transmitted in 

 any other way than by organic union. On the other hand, in 

 the peach yellows group, though no other method of trans- 

 mission has been artificially accomplished than by organic 

 union, there is the clearest possible evidence that natural spread 

 occurs freely, and my own experiments in the case of spike 

 disease of sandalwood lead me to believe that insects will 

 ultimately be found to be responsible. In the other two groups, 



