The "potato" is an enlarged underground stem provided with numerous buds similar to those on the stems of plants above ground. These buds are placed spirally on the underground stem or tuber with a considerable degree of uniformity. As on the stems of other plants, the buds are less numerous and weaker at the base and most numerous and vigorous at the top or upper end. On a smooth well developed long potato, the spiral arrangement of the buds may be illustrated by sticking a tooth-pick or pin in each eye, beginning at the base or stem end, and connecting the pins with a string (Fig. 274). [Illustration: Fig. 274. How to illustrate the spiral arrangement of the eyes.] FARM NOTES ON THE POTATO. Now that we have seen the potato growing in the school-room, some information may be given respecting its treatment in the field as a crop. Potatoes are easily raised, even under adverse conditions, although they respond quickly to superior fertility and tillage. The average yield in the United States during the last ten years was 76.6 bushels an acre, although from three to four hundred bushels an acre are not uncommon under superior tillage when soil and climate are at their best. The area devoted to potatoes during the last decade was two and a half million acres annually. Potatoes do best on a moderately moist and deep soil and in a climate relatively cool. Since the period of growth is short, varying from three to five months, they should be planted in soil which has an abundance of readily available plant-food. Notice in Fig. 272 that most of the underground stems which have produced potatoes leave the main stem about four inches below the surface and but a short distance above the seed-piece. This suggests that the seed should be planted about four inches deep. To produce three hundred bushels of potatoes requires the exhalation of over three hundred tons of water: therefore water or moisture is of quite as much importance in securing large yields as plant-food. It is best to prepare the land deeply, to plant deep, and then to practice nearly or quite level culture. The practice of hilling up potatoes, so common in most parts of the country, is to be discouraged, usually, because it is wasteful of moisture and the tubers do not grow in the coolest part of the soil. For very early potatoes, hilling-up may be allowable. Till the soil very often to save the moisture. For the philosophy of this, see Leaflet No. IX.