264 Journal of Agricultural Research voi. xvii, No. 6 



over the bench, all four treatment groups thus being mixed and appar- 

 ently exposed to similar conditions of light, temperature, and humidity. 

 Each plant had enough space so that it was not in contact with any 

 other. Soil fertilization and watering were similar for all plants. There 

 was as much variation in the amount and type of soil used for the plants 

 that showed mosaic symptoms as for the others. The untreated controls 

 came from neither eye-end sets nor stem-end sets. Each tuber was 

 cut with a flamed knife, and the seed pieces were planted in steam- 

 sterilized soil. Finally, the objection that the method used for intro- 

 ducing aphids brought in the factor of contact with diseased leaves, is 

 met by the results of 14 checks in another room of the greenhouse. 

 These 14 tubers from the same stock produced tuber hills, each of which 

 remained entirely health}^ for 38 days after a mosaic leaf or shoot had 

 been placed upon it when it was 8 inches high. 



In connection with the experiment reported in Table VI another 

 aphid experiment was conducted at Washington, D. C, in the winter 

 of 1918-19, but withaGreen Mountain lot and with insect cages emplo3'ed. 

 From this stock for the last three seasons the mosaic plants had been 

 eliminated, so that but 13 per cent of the plants developed mosaic as 

 soon as new leaves were formed. In this experiment, plants from 5 

 different tubers were used. Each of these tubers was halved, making 

 10 sets. Plants from 5 different sets, designated 474X, 48 ix, 486X, 

 470X, and 478X, were kept in the greenhouse without a cage; and plants 

 from the corresponding 5 different sets, designated as 474y, 481 y, 486y, 

 47oy, and 4783^, were placed in two cages which were kept in the same 

 greenhouse with the uncaged plants. Three of these plants, 474y, 48 ly, 

 and 486y, when from 3 to 6 inches tall were placed in one cage, while 

 the two remaining plants, 47oy and 4783', were placed in another cage. 

 On February 26, 191 9, a few hundred aphids taken from health3' Green 

 Mountain plants were transferred to each of plants designated 47oy 

 and 478y, and similar transfers were made on March i and on March 15. 

 The aphids were brushed upon cardboard with a camel's-hair brush 

 and then transferred to the plants. Before the transfers were made 

 on March 15 the plants were fumigated. Upon 4743% 4813% and 486y 

 aphids from mosaic plants were transferred in a similar manner, but 

 with a different brush and cardboard. Here also three distinct trans- 

 fers were made on February 26, March i, and March 15. The plants 

 were fumigated on March 15 and the third transfer made. At this 

 time a few hundred aphids were placed upon each of the plants. This 

 last set of aphids was allowed to feed on the plants until March 22, when 

 the lower half of each stalk had become defoliated. Then another 

 tobacco fumigation was applied, and the cages were removed from the 

 plants. At this time a few of the newly formed leaves showed distinct 

 mottling. On April 2 newly formed leaves on all 7 stalks representing 

 the 3 different plants were distinctly mottled (PI. 30, A). At this time 



