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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 potatoes is about 131° F. In 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 saying 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,' 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.

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FIG. 5-PLANT OF VERMONT GOLD COIN (VIEWED FROM ABOVE) Grown under field conditions, 1904. There are ten good-sized tubers, weighing 3%2 pounds. The space occupied by the tubers averaged about ten inches on all sides from the center. In this variety, during the past year, the

tubers were distributed all round the stem. In some varieties they tend to form on one side only. FIG. 6—PLANT OF VERMONT GOLD COIN (SIDE VIEW) Same plant as in Fig. 5, showing how completely the soil is filled with roots. The tubers were

with roots. The tubers were all well below ground, the majority being between two and five inches. The roots extended outward two feet

e roots extended outward two feet from the centre of the plant, and downward to a depth of over three feet. Observe the potato bug about seven in

potato bug about seven inches below the surface. It was uncovered at this place.

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a depth of two and a half feet. The deep-growing roots are very tender and brittle and easily broken, differing in this respect from corn roots. The horizontal roots send out vertical branches, which often descend to a depth of two feet or more.

Shallow tillage, such as hand-hoeing without hilling, retains all the roots. Moderately deep tillage with a five-tooth single horse-cultivator and slight hilling destroys practically all the surface roots, and undoubtedly interferes seriously with the plant's development; while with deep tillage nearly all the long horizontal roots are destroyed, and with them all their numerous vertical branch-roots with their intricate system of fibres and root hairs, by which the potato receives its food. In very heavy soils it may be wise to plant potatoes shallow and then hill them, but in most soils it is better policy to plow deep, plant fairly deep, and give shallow flat cultivation. With deep tillage the roots nearest the surface were at a depth of seven inches, while in the case of those receiving shallow tillage the bulk of the horizontal roots were in the surface seven inches. The hilling covers the potatoes and prevents them from sunburning, and this seems to be all the benefit received. · The loss of roots is very hurtful, and takes place at a time when the plant can least afford to suffer injury. Experiments conducted at Vermont Experiment Station' show that during the last weeks of growth the weekly increase in weight of tubers is at its maximum, and that checks when the tubers are approaching maturity depress the yield cor. respondingly. 1 Ver. Bul. 72, p. 5.

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