Mr John Shirreff (in thesame volume, p. 60.) takes a general and philosophical view of the subject, applying to the potato the doctrine by which Mr Knight had accounted for the disappearance of the fine cider fruits of the 17th century. The maximum of the duration of the life of every individual, vegetable as well as animal, is predeter mined by nature, under whatever circumstances the in dividual may be placed : the minimum, on the other hand, is determined by these very circumstances. Admitting, then, that a potato might reproduce itself from tubers for a great number of years in the shady woods of Peru, it seems destined sooner to become abortive in the cultivated champaign of Britain ; insomuch, that not a single healthy plant of any sort of potato that yields berries, and which was in culture twenty years ago, can now be produced. Mr Shirreff concludes, therefore, that the potato is to be considered as a short-lived plant, and that, though its health or vigour may be prolonged, by rearing it in ele vated or in shady situations, or by cropping the flowers, and thus preventing the plants from exhausting them selves, the only sure way to obtain vigorous plants, and to ensure productive crops, is to have frequent recourse to new varieties raised frown the seed. The same view, it may be remarked, had occurred to Dr Hunter, who, in his Gcorgical Essays," limits the duration of a variety in a state of perfection to about fourteen years. A fact ascer tained by Mr Knight deserves to be particularly noticed : it is this ; not b) planting late in the season, perhaps in June or even in July, an exhausted good variety may in a great measure be restored ; that is, the tubers resulting from the late planting. when again planted at the ordinary season, produce the kind in its pristine vigour, and of its former size.
Jerusalem ?rtichoke.
312. The Jerusalem .drachoke, or tuberous-rooted sun flower, (Helianthus tuberosus, L. ; Syngenesia Polygamia fruNtranra ; Corymbiferze, Juss.) is a perennial plant, origi natty limo Brazil. It has the habit of the common sun flowe.r, but grows much taller, often rising ten or twelve feet high. Though its roots endure our hardest winters, the plant seldom flowers with us, and it never ripens its seed, The roots are creeping, and have many tubers clus tered together, perhaps from thirty to fifty at a plant. These are eaten boiled, mashed with butter, or baked in pies, and have an excellent relish. The plant was intro duced into our gardens early in the 17th century ; and be fore potatoes became common, it was much more prized than at present. The epithet Jerusalem is a mere corrup tion of the Italian word Girasole, or sunflower ; the name artichoke is bestowed from the resemblance in flavour which the tubers have to the bottoms of artichokes. As the potato is the pomme de terre, this is the /wire de terre of the French.
The plant is readily propagated by means of the tubers. They are cut in the manner of potato sets, and planted, in any light soil and open situation, in the end of March. They ge placed in rows, three feet asunder, and a foot or fifteen inches apart in the rows. In September they are fit for use ; and in the course of November they are dug up and housed, being kept in sand like carrots. Some times they are left in the ground, and dug only as wanted, being best when newly raised. The only disadvantage is, that in this way they cannot be had in severe frosts. It is not very easy to clear the ground of them where they have once grown ; and on this account, some gardeners devote a by-corner to them, and allow them to remain from year to year, taking up only what is wanted for the occasional use of the family. But the tubers thus produc
ed are not so clean or well flavoured as those produced on newly delved ground by yearly planting.
Turnip.
313. The Turnip (Brassica Rapa, L.) is a biennial plant, growing naturally in sonic parts of England, and figured in " English Botany," t. 2176. The root-leaves are large, of •k.e.p green colour, very rough, jagged and gashed. In the second season it sends up a flower-stalk, four or five feet high, having leaves which embrace the stem, very different from the former ; smooth, glaucous, oblong, and pointed. The cultivated variety with a swelling fleshy root has long been known. Of this there are several well marked subvarieties, distinguished as garden or as field To the former belong the Early Dutch, Early Stone, and the Yellow ; to the latter, the Large White, the Globe, the Swedish, the fted•topped, and the Tankard or oblong.
314. For the supply of the table during the early part of summer, some of the early Dutch turnip is commonly sown. If the weather prove dry, regular watering is pro per. For a general crop, the large green-topped white turnip is accounted excellent, as being soft, juicy, and sweet. One of the kinds with which the London market is often supplied is the stone turnip, a hard sweet sort, seldom of a large size. The yellow is now perhaps less cultivated than it formerly was ; but the yellow Dutch may still be considered as one of the best kinds for winter use, as no frost hurts it, and it is of excellent flavour. It is a very distinct variety, the flesh being yellow through out ; whereas, in the other varieties, any difference of colour is only in the rind. The red or purple-topped turnip was formerly much cultivated ; but the green-topped has now in a great measure superseded it, though less hardy. The general crop is often sown towards the end of June, when refreshing showers may be expected. It not uncom monly occupies the ground from which early peas have just been removed. But as turnips are most desirable for the table in a young state, a small sowing is commonly made once a month from April to August. If sown earlier than April, the plants are apt to run to seed. To divide the seed more equally when sown broadcast, a little fine earth is mixed with it in sowing. It is frequently sown in chills, an inch deep, and somett hat more than a foot asunder. If rain do not occur, frequent watering is of great advantage to the young crops. A light sandy loam, not recently manured, is best for turnip ; in a rich garden soil, the roots are apt to become rank and woody. When the root-leaves are about an inch broad, the plants are hoed ; and, if they have been sown broadcast, thinned to six or eight inches distance from each other. When young turnips are daily drawn for the table, they may be allowed to stand some what closer, the proper degree of thinning being accom plished by pulling for use. If sown in drills, they may stand at five inches from each other in the lines. Turnips bear transplantation with difficulty ; yet in moist and rainy weathe•, spots where the seed has failed may be filled up. When showery weather has made the leaves spring too much, so as to threaten the production of a flower-stem, it is sometimes found useful to tread them down, by placing the foot gently on the centre of the plant. In some gar dens, turnips are sown on a slight hot-bed in February, and thus forwarded by two or three weeks ; but the beds must have as much air as can possibly be given.