EXPERIMENT STATION BULLETINS. 445 



present the systematic position of the tomato organism must be left open 

 for further study. 



The potato leaf-roll organism is a protozoan which even more strongly 

 resembles members of the genus Trypanosoma. The sinuous form of the 

 body and the occurrence of an undulating membrane are characters which 

 belong only to organisms of this type. That other forms also occur in the 

 life cycle is verj^ probable. Some of the long slender organisms found in the 

 sieve tubes have a great resemblance to spirochaetes. They are, however, 

 much larger than any kno\^^l species, and slender trypanosomes have often 

 been mistaken for spirochaetes by protozoologists. The recent work of 

 Mesnil with Leptomonas Davidi, a parasite in the latex of Euphorbiaceous 

 plants, has showni that these organisms exhibit great polymorphism, some 

 individuals being typical Leptomonads, while others more nearly resemble 

 trj'panosomes. It is probable, therefore, that the potato leaf-roll organism 

 will be found to contain quite widely varying forms in its life-cycle. 



OTHER KNOWN PROTOZOAN DISEASES OF I'LANTS 



Lafont (6), Franca (7), Mesnil (8), Franchini (9), and other European 

 workers have found protozoa in jilants belonging to Euphorbiaceae, Asclepi- 

 adaceae and Urticaceae. Typical Leptomonad forms have been found in 

 the latex of Euphorbiaceous plants, and these organisms are constantly asso- 

 ciated with a disease of the plants, characterized by etiolation and wilting. 

 Transmission of the flagellates has been shown to occur through the agency 

 of sucking insects (Coreidae). In the bodies of these insects the protozoan 

 passes a portion of its life-cycle, eventually developing non-flagellate forms 

 which are found in the salivary glands. Flagellate Leptomonad and Trjqato- 

 monad forms have been found in the latex of Ficus parietalis, as have also some 

 characteristic amoeboid organisms. 



The measurements of the flagellates from the latex of the Euphorbiaceous 

 plants very nearly coincide with those found in bean plants affected with 

 mosaic. The mono-flagellate character of the organisms from the latex of 

 these plants indicates that these are distinct from the organisms associated 

 with bean and clover mosaic. 



The demonstration of protozoa as the cause of plant disease had its incep- 

 tion in 1910 in the work of Lafont (6) who found the Leptomonad forms in 

 the latex of certain Euphorbiaceous plants. This stands as the first record 

 of a plant disease caused by a protozoan parasite. That a great many 

 other plant diseases of unknowii cause, will, upon investigation, yield similar 

 results, is not unlikely from the evidence now available of the relation of 

 these simple organisms to plant disease. 



PROBABLE RELATION OF THE ORGANISMS TO THE DISEASES AVITH WHICH THEY 



ARE ASSOCIATED 



Since the description of tobacco mosaic by Mayer (15) in 1886, the cause 

 of this and other mosaic diseases has been a question of paramount interest to 

 phytopathologists. There is no longer any doubt of the infectiousness of 

 material from diseased plants, and that the power of such a small amount of 

 material to produce disease when introduced into healthy plants is a char- 

 acteristic of some "living contagium" seems evident from the work of Allard 

 (14) and others. The fact that insects carr}^ and transmit the causative 

 agents of these diseases is itself significant, because of the known facts of 



