Proc. Soc. Hort. Sci. 10: 99-100 (1913)
SOME ECOLOGICAL EVIDENCE OF THE ORIGINAL HABITAT OF THE IRISH POTATO, SOLANUM TUBEROSUM
C. L. FITCH
Iowa State College, Ames, Iowa.

There is at this time a great deal of interest in the breeding and introduction of new varieties of potatoes and in tracing our common commercial sorts back to their original sources.

Some recent physiological and ecological studies by the writer are offered as suggestive of the type of structure desirable (if such structures are in existence) in a potato adapted to the corn belt, a territory which has a comparatively warm dry air and a relatively close grained subsoil. These studies also afford significant evidence of the conditions under which in South America we might expect to find in their original habitat, the wild Solanums that produced our cultivated sorts.

The structure of the stoma of the potato leaf would seem to indicate that the plant came from a region of moist atmosphere in that there is no special means of protection against the loss of moisture during respiration, but rather the readiest access afforded the outer air to reach the moist cell walls within the leaf. The stoma is of a form to facilitate the escape of moisture rather than to protect against the loss of water. The stoma projects like a low crater and is unguarded by the devises often found on other plants. The stoma, for instance, of the alfalfa, which we know to be native to an arid climate, is sunken and provided with a shielding ring or awning above the guard cells. In connection with the evidence of the stoma, we may note also that the chief fungous enemies of potato leaves, early and late blight, both flourish in a damp atmosphere.

The structure of the root cap is likewise significant. The cells seem to have no coherence and are readily broken from one another by agitation in water or by slight abrasion. Whether taken from a sprouting seed, from a root in the field, or from a pot in the greenhouse, I have found the cap hardly able to hang together and readily broken up by dragging on soft, wet filter paper. This is such a root cap as might be produced by growing for untold ages in soils so loose and open that the plant root had little or no need for a protecting cap. The conclusions from the physiological needs of the potato roots for cool and open soil, as expressed quantitatively in experiments reported by the writer to this society last year, and the conclusions from the structure of the root cap, are the same. In studies of potato root systems in widely different soils and in several varieties, I have found that extreme crookedness of growth is characteristic, and that in a half dozen potato districts of Colorado, potato roots do not penetrate raw clay, though they may go down a rotten sage brush root and send out branch roots into the sandy streaks between the layers of clay.

In rather heavy loams, potato roots do not go down alfalfa roots when the sod was broken in the spring, but when the alfalfa had been broken the year before and the alfalfa roots had commenced to decay and to pack their holes less tightly I found potato roots following alfalfa roots down through clay for as much as four feet. On much of the successful potato soil of the Carbondale district potato roots do not penetrate, unless preceded by alfalfa, for more than two feet, and in my judgment, potato growing on such soils would be impossible except for the excellent drainage secured by the steep slope of the mountain mesas.

If we compare the potato root habit and root cap with the root habit and root cap of the alfalfa, we find a striking contrast. The alfalfa root goes straight down through raw clay soils unpenetrated before by any root, and the alfalfa root cap is snug and loses very few cells when subjected to rubbing, but seems rather to cover the root like a glove finger.

Physiologists and ecologists regard these needs and structures of potato roots as evidence of the conditions under which the plant has grown for ages past. We may apply these facts to the examination of the claims that certain localities in South America are the original habitat of our varieties of potatoes.

It is commonly accepted that the potato is native to South America and the question has arisen as to whether its native habitat is to be found in the maritime districts or in some of the more arid Andean regions. The physiological and ecological data outlined above are submitted as one source of evidence in answer to this question. To the writer they clearly suggest its being a plant native to sandy open soils and maritime atmosphere, or misty well drained mountain slopes free from prolonged and heavy rains, if such there be.