This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
The manufacturing processes of the potato plant are described in " Bulletin 71" of the Wyoming Experiment Station, as follows:
In order to understand the relation of the leaves to the tubers it is necessary to know that the starches and other food materials which are stored up in the tubers are produced within the leaves through the activity of the contents of the leaf cells when influenced by the action of light. The leaves are green because the cells contain the green bodies technically known as 'chloroplasts.' No plant which lacks' chloroplasts' is capable of manufacturing starch.
Leaf structure is essentially the same in all plants. A section from the upper to the under side will show on either side an epidermis of flattened, colorless cells. The cells immediatly underneath the upper layer are elongated and closely packed and are known as the palisade tissue. The lower half of the leaves contain nearly spherical cells, rather loosely arranged, with conspicuous air spaces near the lower epidermis. These communicate freely with the smaller spaces among the cells, and are continuous one with the other throughout the leaf. The air spaces are in direct communication with the outside air through tiny openings known as stomata. These openings on the surface often occur on both sides of the leaf, but in most crop plants they are more numerous on the under side, or may be entirely wanting on the upper. They are very important to the plant, since it is through them that the plant food derived from the atmosphere finds ingress to the leaf, which is really the plant's starch factory. They also serve the very important function of allowing the escape of the watery vapor and the oxygen which is released during the period of active starch formation. Necessary as are these openings on the leaf surface, they are also sources of great danger to the plant. The rest of the surface of the leaf is so thickened or waterproofed with the substance known as cutme that water cannot enter nor escape through it, neither can germs from the atmosphere find entrance into the leaf. When germs or other fungi secure admission to the tissues of the plant it is usually either through the stomata or through wounds upon the leaf or other part of the plant. It is for this reason that insect attacks are so often followed by fungous and germ diseases. Similarly wounds which plants accidentally receive or which are made by pruning are also often followed by diseased conditions.
The crude sap of plants, which is essentially the soil water with the gases and soluble minerals of the soil dissolved in it, is taken up by the roots and conveyed through the stem to the leaves. The air which has been admitted to the leaf through the stomata supplies the carbon dioxide.
From these ingredients, through the agency of the protoplasm of the cell and the 'chloroplasts' acting in the presence of sunlight, starch is manufactured. 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 transmitted 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 injury to the bark will prevent the formation of tubers, as the sap conveying the soluble starch can no longer 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."