Sweet potatoes are grown in Africa; but either the cultivation is rare, or the species are different. Robert Brown[165] says that the traveller Lockhardt had not seen the sweet potato of whose cultivation the Portuguese missionaries make mention. Thonning[166] does not name it. Vogel brought back a species cultivated on the western coast, which is certainly, according to the authors of the _Flora Nigritiana_, _Batatas paniculata_ of Choisy. It was, therefore, a plant cultivated for ornament or for medicinal purposes, for its root is purgative.[167] It might be supposed that in certain countries in the old or new world _Ipomœa tuberosa_. L., had been confounded with the sweet potato; but Sloane[168] tells us that its enormous roots are not eatable.[169] _Ipomœa mammosa_, Choisy (_Convolvulus mammosus_, Loureiro; _Batata mammosa_, Rumphius), is a Convolvulaceous plant with an edible root, which may well be confounded with the sweet potato, but whose botanical character is nevertheless distinct. This species grows wild near Amboyna (Rumphius), where it is also cultivated. It is prized in Cochin-China. As for the sweet potato (_Batatas edulis_), no botanist, as far as I know, has asserted that he found it wild himself, either in India or America.[170] Clusius[171] affirms upon hearsay that it grows wild in the new world and in the neighbouring islands. In spite of the probability of an American origin, there remains, as we have seen, much that is unknown or uncertain touching the original home and the transport of this species, which is a valuable one in hot countries. Whether it was a native of the new or of the old world, it is difficult to explain its transportation from America to China at the beginning of our era, and to the South Sea Islands at an early epoch, or from Asia and from Australia to America at a time sufficiently remote for its cultivation to have been early diffused from the Southern States to Brazil and Chili. We must assume a prehistoric communication between Asia and America, or adopt another hypothesis, which is not inapplicable to the present case. The order _Convolvulaceæ_ is one of those rare families of dicotyledons in which certain species have a widely extended area, extending even to distant continents.[172] A species which can at the present day endure the different climates of Virginia and Japan may well have existed further north before the epoch of the great extension of glaciers in our hemisphere, and prehistoric men may have transported it southward when the climatic conditions altered. According to this hypothesis, cultivation alone preserved the species, unless it is at last discovered in some spot in its ancient habitation—in Mexico or Columbia, for instance.[173] +Beetroot+—_Beta vulgaris_ and _B. maritima_, Linnæus; _Beta vulgaris_, Moquin.