THE POTATO " 531 



From these ingredients, through the agency of 

 the protoplasm of the cell and the 'chloroplasts' 

 acting in the presence of sunlight, starch is manu- 

 factured. This starch is temporarily stored in 

 the leaf, but at night, when starch formation no 

 longer is going on, the starch is, through the action 

 of a ferment, converted into soluble form and is 

 transmitted from the leaf through the stem to the 

 underground stems, which become gorged with 

 the material thus received. Another ferment 

 now reconverts this soluble form of starch into 

 the insoluble, which is then deposited in the tuber 

 as a permanent part of it. The ultimate size of 

 the tubers, therefore, depends upon the amount of 

 starch that is thus formed by the leaves and trans- 

 mitted from time to time to the tubers for storage. 

 "It seems that most of the crude sap passing 

 upward in the stem is transmitted through ducts 

 in the interior tissue, while the elaborated sap (as 

 it is called when it has been acted upon by the 

 leaf and is in condition to be used as plant food) is 

 conveyed downward close to the exterior of the 

 stem, viz., in the interior layers of the bark. It 

 follows, therefore, that any injury to the inner 

 bark or any artificial obstruction in these layers 

 will prevent the downward movement of this 

 elaborated sap. In the case of the potato, an in- 

 jury to the bark will prevent the formation of tubers, 

 as the sap conveying the soluble starch can no lon- 

 ger reach them. In the event of the elaborated 

 sap being left in the above ground portions, it results 

 either in very marked increase in the size of leaf 

 and stem, through forced feeding, or the plant will 

 attempt the formation of tubers above ground." 



