THE POTATO 279 



were tested separately, and in each case it was 

 found that the percentage of starch in the pronged 

 tubers was smaller than in the regular tubers. 

 This seems to be one of the causes of the variation 

 in the starch content above referred to. 



"A test of different-sized tubers of the same 

 variety proved that there was practically no differ- 

 ence in the starch content of large and small tubers. 



*'In studying the influence of the depth at 

 which tubers grow in the soil upon the starch con- 

 tent, it was found the first year that in every case 

 the percentage of starch was largest in the deeper- 

 growing tubers and smallest in those growing 

 nearest the surface. When we consider the slight 

 variation in depth at which the tubers grow in the 

 soil these facts are significant. The next year the 

 experiment was repeated with a trial of level and 

 hill culture. The level culture gave higher starch 

 content than hill culture, and the variations with 

 the depth were greater in the hill culture than in 

 the level culture. These facts suggest a possible 

 explanation of the depth influence — viz., that it 

 acts through the temperature of the soil. The 

 deeper tubers are in a cooler medium than the 

 shallow ones, and soil that is hilled is warmer in 

 warm, dry weather than that which is not hilled. 

 The variation of temperature in the deeper and 

 shallower layers would naturally be greater in the 

 hilled soil. 



"In experiments in planting at different dis- 

 tances the starch content increased as the distance 

 between the plants decreased. This seems to ac- 

 cord with the results of the tests of depth of plant- 

 ing, since close planting promotes shading of the 

 ground and thus tends to reduce the soil tempera- 

 ture. 



