876 



SCIENCE 



[N. S. Vol. XXXVI. No. i 



tensity and distribution of the pink coloration 

 of the corollas in much the same manner that 

 it affects the intensity and distribution of the 

 green color of the leaves. For this reason an 

 examination of the blossoms often affords one 

 of the surest indications of the presence of 

 mosaic in a tobacco plant. So far as the 

 writer is aware, this distinctive appearance of 

 the blossoms has not before been mentioned. 



Specific Infection Apparently Responsible for 

 the Development of Mosaic 



The writer's experiments do not bear out 

 the conclusions of earlier investigators that a 

 true, infectious mosaic can be produced in 

 plants by simply cutting them back. 



If the infectious principle of mosaic is not 

 accidentally introduced during the course of 

 the experiments, and if the plants are abso- 

 lutely free from the disease at the time of 

 cutting, our experiments indicate that these 

 plants may be cut back indefinitely without 

 producing mosaic. Healthy plants have been 

 constantly cut back, in some instances for 

 long periods, without producing any symptoms 

 of true mosaic. This was not accomplished, 

 however, until thorough methods of sterilizing 

 were observed, and the plants had been care- 

 fully screened and fumigated to exclude 

 aphids. 



Boil Infection 



All experimental data at hand indicate that 

 soil infection is not as important a factor as 

 has been supposed in the origin of mosaic in 

 the seed bed. In many experiments it has 

 been shovsoi beyond question that an outbreak 

 of mosaic in young plants was not in any way 

 connected with soil infection. 



Aphids as Carriers of the Mosaic Disease 

 In the course of the writer's greenhouse ex- 

 periments with tobacco, it was observed that 

 the occurrence of mosaic in plants at various 

 stages of development was in some manner 

 frequently associated with aphid infestation. 

 Under control conditions it was observed 

 that healthy tobacco plants were always ob- 

 tained if these were grown in screened cages 



fumigated at short intervals to prevent aphid 

 infestation. On the other hand, when colonies 

 of aphids present in the greenhouse were in- 

 troduced into other cages containing healthy 

 plants, infestation of the plants resulted in a 

 wholesale occurrence of mosaic. This phase 

 of the question has been taken up in coopera- 

 tion with the Bureau of Entomology. 



Following inoculation a somewhat variable 

 inoculation period preceded the first symptoms 

 of mosaic. This period and the subsequent 

 symptoms appear to be practically the same 

 in whatever manner the disease may be com- 

 municated to healthy plants. 



In the light of the facts brought out in our 

 experiments, it is not easy to see how the 

 mosaic disease of tobacco can be logically 

 placed in the category of purely physiological 

 diseases. These facts strongly suggest the 

 presence of a living, active microorganism. 



It has been suggested at various times that 

 the pollen grains may be the carriers of a 

 mosaic disease. Earlier investigators have 

 shown (and the writer has substantiated these 

 results) that the seed of mosaic plants pro- 

 duces healthy plants. As a matter of fact, 

 the general rule of self-fertilization of the 

 tobacco flower means that the pollen grain and 

 ovules are both produced by a mosaic plant. 

 These facts are not favorable to the pollen- 

 grain transmission of the disease. 



H. A. Allard 



Btjeeau of Plant Industry, 

 Washington, D. C. 



TBE CONVOCATION WEEK MEETING OF 

 SCIENTIFIC SOCIETIES 



The American Association for the Advance- 

 ment of Science and the national scientific 

 societies named below will meet at Cleveland, 

 Ohio, during convocation week, beginning on 

 December 30, 1912. 



American Association for the Advancement of 

 Science. — President, Professor Edward C. Picker- 

 ing, Harvard College Observatory; retiring presi- 

 dent, Professor Charles E. Bessey, University of 

 Nebraska; permanent secretary, Dr. L. 0. Howard, 

 Smithsonian Institution, Washington, D. C. ; gen- 



