V64: POTATO duced. 19. There are three citrates, tripotas- sic, dipotassic, and monopotassic citrate. Tri- potassic citrate, citrate of potassium, or com- mon citrate of potash, KaCeKUO? + H 2 O, is formed when a solution of citric acid is neu- tralized by potassic carbonate. On evaporation it crystallizes in transparent stellate-grouped needles, which lose their water of crystalliza- tion at 392 F. They are very deliquescent, and insoluble in alcohol. Citrate of potash is adopted as a refrigerant and diaphoretic in the United States and British pharmacopoeias, and is sometimes prepared extemporaneously as an effervescent draught. Dipotassic citrate, KaCeHeOT, made by adding an equivalent of citric acid to the tripotassic salt and evapora- ting, and the monopotassic citrate, KCat^O?, are both acid salts, the latter crystallizing with 13*8 per cent, of water of crystallization and having an agreeable sour taste. Malate of po- tassium exists to a certain extent in the juice of some fruits, but malic acid more commonly exists in combination with calcium. POTATO (Span, batata, the name for sweet potato, erroneously transferred to a very dif- ferent plant), the plant and tuber of solarium tuberosum. The genus solarium contains over 900 described species; it includes annual and perennial herbs, shrubs, and even trees, which, though widely distributed, are more abundant in tropical South America than elsewhere ; it is the typical genus of a large and important family, the solanacece, in which there are about 60 genera. Tobacco, stramonium, belladonna, and henbane are powerful narcotics belonging to the family ; and besides the potato, it fur- nishes as esculents the tomato, egg plant, cap- sicum, and physalis or winter cherry. The leading characters of the genus solanum are a five-parted calyx ; a five-lobed, wheel-shaped corolla, with scarcely any tube ; stamens five with very short filaments, their anthers con- verging to form a cone around the pistil, each anther cell opening by a pore at the top ; ova- ry two-celled, with a simple style, and in fruit becoming a two-celled, many-seeded berry. There are several species belonging to the tuber-bearing section, two of which, 8. Fend- leri and 8. Jamesii, are found in the moun- tains of New Mexico. The potato is one of the few generally cultivated plants that are well known in the wild state, and concern- ing the origin of which there is little room for discussion. There is abundant evidence that it grows wild at the present day in Peru and Chili, on the island of Chiloe, and else- where; it is probable that there are four or five varieties in the wild state, and not un- likely that some of those described as dis- tinct tuber-bearing species are forms of 8. tu- berosum. The potato was carried to England in Sir Walter Raleigh's vessels from Virginia in 1586; but there is good reason to believe that it had been introduced into Spain much earlier from Quito ; in 1588 it was sent to Flanders from Italy, where it had been re- ceived from Spain, and was at that time a common article of food, and was even fed to the pigs. Moreover, it is thought that its oc- currence in Virginia was due to a recent in- troduction by the Spaniards ; there is no proof that it was in cultivation by the aborigines of this country or those of Mexico. After its in- troduction into Europe the potato made very slow progress, and it is only within 100 years that its cultivation has been common even in Ireland, a country which since then has so largely depended upon it. In the most im- portant gardening work of its time, published in 1771, only two varieties of the potato are mentioned, a white and a red. Though popu- larly called a root, the tuber of the potato is really an underground stem (see PLANT), en- larged by the accumulation of starch, which is there stored up for future use. The true na- ture of the tuber may be best seen by care- fully taking up a young plant just as the pota- toes are beginning to form ; besides the prop- er roots there will be found stems, longer or shorter according to the variety, the ends of which and their branches have begun to swell to form tubers; usually a plant will show every gradation, from the merest swelling to well formed if not large tubers. The leaves upon these underground branches are represented by scales, often conspicuous when the tuber is young, and when it is full grown appearing as a distinct scar. In the axils of these rudi- mentary leaves are found the buds for next year's growth, popularly called the eyes of the FIG. 1. Tubers in different stages of development. potato, and these may consist of a central bud with several accessory buds on each side of it, or, in some varieties with prominent eyes, as a sort of suppressed branch with buds crowded
