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Journal of Agricultural Research voi.xvn.No. 6 



and in addition each tuber was split lengthwise so that each seed piece 

 included eyes from the bud-end and the stem-end. Also, no mosaic 

 leaves were put in contact with the plants when aphids were introduced, 

 and all cages that had been used previously were steam sterilized. 



PHYSIOLOGICAL STUDIES 



Some work on the chemical differences between the healthy and dis- 

 eased potato plants was carried out in connection with this investiga- 

 tion. These experiments included a determination of the reducing and 

 total sugars and starch content in the healthy and diseased leaves of 

 potato plants grown under the same environmental conditions. In this 

 work the potato leaves were picked off the stems and weighed. They 

 were dried in a hot oven to constant weight and extracted with alcohol 

 in a Soxhlet extractor. The sugars were determined in the extract, and 

 the starch in the residue. The results of these determinations are given 

 in Table VII. Data are given in this table as to the location of the 

 plants from which the samples were taken, whether caged or in the open, 

 the time of sampling, and whether the day was bright or cloudy. In 

 the other columns of the table are given the results of the sugar and 

 starch determinations. The determinations compared in the table are 

 from plants grown under as nearly the same conditions as possible. 



TablS VII. — Sugars and starch in healthy and mosaic Green Mountain potato foliage 



From the results shown in Table VII it appears that mosaic plants 

 have a higher sugar content than the healthy plants grown under the same 

 conditions. This is true of both reducing and nonreducing sugars, 

 though the differences in the latter are not so marked. There is an 

 average of about i per cent more total sugar in the mosaic plants than 

 in the healthy. With starch this relation is reversed, healthy plants 

 having an average of about 4 per cent more starch than those affected 

 with mosaic. It is, of course, to be remembered that the investigations 



