[169] Several Convolvulaceæ have large roots, or more properly root-stocks, but in this case it is the base of the stem with a part of the root which is swelled, and this root-stock is always purgative, as in the Jalap and Turbith, while in the sweet potato it is the lateral roots, a different organ, which swell.
[170] No. 701 of Schomburgh, coll. 1, is wild in Guiana. According to Choisy, it is a variety of the Batatas edulis; according to Bentham (Hook, Jour. Bot., v. p. 352), of the Batatas paniculata. My specimen, which is rather imperfect, seems to me to be different from both.
[171] Clusius, Hist., ii. p. 77.
[172] A. de Candolle, Géogr. Bot. Raisonné, pp. 1041-1043, and pp. 516-518.
[173] Dr. Bretschneider, after having read the above, wrote to me from Pekin that the cultivated sweet potato is of origin foreign to China, according to Chinese authors. The handbook of agriculture of Nung-chang-tsuan-shu, whose author died in 1633, asserts this fact. He speaks of a sweet potato wild in China, called chu, the cultivated species being kan-chu. The Min-shu, published in the sixteenth century, says that the introduction took place between 1573 and 1620. The American origin thus receives a further proof.
[174] Moquin-Tandon, in Prodromus, vol. xiii. pt. 2, p. 55; Boissier, Flora Orientalis, iv. p. 898; Ledebour, Fl. Rossica, iii. p. 692.
[175] Roxburgh, Flora Indica, ii. p. 59; Piddington, Index.
[176] Theophrastus and Dioscorides, quoted by Lenz, Botanik der Griechen und Römer, p. 446; Fraas, Synopsis Fl. Class., p. 233.
[177] Heldreich, Die Nutzpflanzen Griechenlands, p. 22.
[178] Alawâm, Agriculture nabathéenne, from E. Meyer, Geschichte der Botanik, iii. p. 75.