eyes, if desired, and a shoot obtained from each half. As each shoot sets a root it may be broken off and transplanted, and another may start. By these means and great care a pound of seed tubers has been made to yield 2,558 pounds of potatoes in one season.

Historical Note.-The early history of the potato is obscure. The most authentic information I have secured is that Sir Robert Southwell, the President of the Royal Society of England, at the meeting held December 13, 1693, stated that the potato was brought into Ireland by his grandfather, who obtained tubers from Sir Walter Raleigh, after the return of his expedition from Virginia. This was in the year 1584. It is now believed that Sir Walter Raleigh fitted out this expedition, but did not lead it personally, and never was in Virginia. Timbs' "Curiosities of History," page 233, places the date of its introduction to the British Isles as 1586.

CHAPTER II

SOME CONDITIONS INFLUENCING GROWTH

AND DEVELOPMENT

It is common knowledge that a certain amount of heat and an adequate supply of air and moisture are essential for plant growth. All plants that have green leaves require light, in addition, to enable them to assimilate carbon dioxid from the air, dissociate it into its component parts, and elaborate the carbon into such complex substances as starch, sugar, and other carbohydrates.

Influence of Light on Yield.-E. Pagnoul' placed colored glass over different potato plants. Two plants under darkened glass elaborated 31 and 20 grams of starch respectively, while those under ordinary glass elaborated 170 and 110 grams; at the same time plants under normal conditions elaborated 223 and 361 grams. To the favorable influence of abundant light this writer attributes the large yield of potatoes in a season when the aggregate number of hours of sunshine is unusually large. At Wisconsin Experiment Station coldness and cloudiness were believed to be the causes of a poor yield.'

The Amount of Moisture.-The amount of water the plant can obtain from the soil is closely cor

1 E. S. R., V., p.116. 2 Wis. Report, 1902, p. 188.

related with the mode of development. If the soil is very dry, and particularly if the tuber is cut, the seed tuber may be so weakened by loss of moisture that it cannot grow. If a tuber has access to but a small amount of water, there will be little or no root development, with little formation of leaf shoots, but tubers will be formed. Advantage is taken of this fact when small early potatoes are required, the tubers being placed in sand, in a cellar, when small tubers will form, but none or few leaves. Under certain conditions, with an abundance or excess of moisture, numerous leaf shoots and roots appear, but no tubers. An increase in the supply of moisture in the air has been found to favor the development of leaves on the shoots, where only scales were formed in an insufficient supply of moisture.

Respiration. We may say that all plants breathe or take in oxygen and give off carbon dioxid. With potatoes this is a necessary function, and if checked, growth is injured. It is probable that light induces some conditions more favorable to increased respiration than darkness; hence, if the object is to store potatoes, it will be better to hold respiration at its lowest point and keep them in the dark. Respiration cannot go on without force or energy, and as this must be supplied, at least partly, from the tuber, it follows that active respiration will be attended by loss of weight, and this goes on very rapidly when the tuber sprouts. If we wish to "sprout" tubers, the best conditions for doing so are still undetermined.

Influence of Temperature on Respiration.All plants have a range of temperature at which respi

nate Pat

a Thermota

in soil. toes is about 131° F.

ration is normal. The minimum, optimum, and maximum temperatures have been ascertained for some plants. Young wheat plants will respire at as low a temperature as 28° F., or below freezing-point. The optimum temperature for wheat is about 104° F., while that of potato plants is about 113° F. The maximum for wheat is 113° F., while that for potaIn other words, the potato respires best at about 113° F., but should the temperature go above 131° F., the respiration will be somewhat less than before, and the vitality weakened; hence, after a hot spell, when the temperature exceeds the maximum for respiration, it is noticeable that the potatoes fail and become more susceptible to the blight or other troubles, owing to their impaired constitution. By selection we might procure plants of greater vitality, capable of standing the higher temperatures, which would enable them to be better " disease-resisters." Present-day potatoes thrive best in a cool climate.

Influence of Temperature on Growth. The minimum temperature for germination of potato tubers is about 50° F.; hence, in the Northern States early planted tubers make little or no growth unless planted shallow, and this is not desirable, except, perhaps, for the earliest varieties. It is better to germinate the tubers in the barn before planting, thus saving time (see Chapter VI., "Sprouting Potatoes").

Potato Roots.-Generally speaking, far more attention has been paid to the stems and leaves of plants than the roots, yet in order to cultivate the soil in a rational manner it is essential to know where the roots

are, their character, and requirements. Examination of the roots of Early Ohio potatoes,1 made July 5, 1899, forty-three days after planting, about the time the crop received its third cultivation, showed that at this time there was little growth of fibrous roots-only the skeleton system supplied with numerous delicate root hairs. The seed tuber appeared to be sound and whole, but on closer examination it proved to be but a shell. Only a few eyes on the upper side of each tuber produced shoots; thus one hill produced three stalks from two eyes, and another had seven stalks springing from five eyes. The latter plant had more numerous but smaller roots than the former. Twenty-five small potatoes were set on the first plant, the largest of which were the size of a large pea. At this stage of development the main portion of the roots was in the surface eight inches, a few roots reached to the depth of eighteen inches, but the greatest root growth was in a horizontal direction. The roots from each hill had already met and interlaced, some having reached a length of two feet, the plants being three feet apart. At six inches from the hill some of the main lateral roots were but two and one-quarter inches from the surface of the ground, while midway between the rows their depth was barely three inches from the surface.

Further examination of Early Ohio potatoes seventytwo days after planting, when the tubers were nearly full size, showed that the main root growth was in the upper foot of soil; several of the large horizontal roots were within three inches of the surface, and one was but one inch deep. Some of the vertical roots reached

1 N. Dak. Bul. 45, P. 541.