APPENDIX. 105 



action of the sun's rays on the soil is prevented by 

 the thick foliage, and thus the drying up of the soil 

 and its injurious consequences are avoided. 



" The checking of the transformation in the leaf 

 is equivalent to the interruption of the natural 

 change of the leaves into calyces, corollae, stamens, 

 and pistils, which is effected at the expense of the 

 nutrient matters collected in the plant ; and these, 

 when this modification of the leaves is arrested, are 

 turned to account in the formation of tubers. 



" Led by these views, I made, in 1846, experi- 

 ments on single potato plants, carefully marked, by 

 pinching off the ends of the branches. They were 

 so readily distinguished in their subsequent growth 

 from the plants beside them, by more numerous 

 branches, larger and darker foliage, that in truth no 

 marking was necessary. 



" The produce from these plants of tubers was 

 abundant, and the tubers were perfectly healthy; 

 while the plants next them which had not been so 

 treated, gave uniformly a less produce, at the same 

 time the tubers were rough on the surface, and in 

 many instances attacked with the prevailing disease. 

 This experiment was incomplete, and did not give 

 a positive result, but it was yet encouraging for me. 



" In the middle of April, 1847, an experiment 

 was made on a low-lying field with the round white 

 potatoes, generally cultivated here, a variety which 

 had not suffered much from the disease which first 



