The botanical distinction of the species has always presented difficulties, because the male and female flowers are on different individuals, and because the characters of the rhizomes and the lower part of the exposed stems cannot be studied in the herbarium. The last complete work is that of Kunth,[296] published in 1850. It requires revision on account of the number of specimens brought home by travellers in these last few years. Fortunately, with regard to the origin of cultivated species, certain historical and philological considerations will serve as a guide, without the absolute necessity of knowing and estimating the botanical characters of each. Roxburgh enumerates several _Dioscoreæ_[297] cultivated in India, but he found none of them wild, and neither he nor Piddington[298] mentions Sanskrit names. This last point argues a recent cultivation, or one of originally small extent, in India, arising either from indigenous species as yet undefined, or from foreign species cultivated elsewhere. The Bengali and Hindu generic name is _alu_, preceded by a special name for each species or variety; _kam alu_, for instance, is _Dioscorea alata_. The absence of distinct names in each province also argues a recent cultivation. In Ceylon, Thwaites[299] indicates six wild species, and he adds that _D. sativa_, L., _D. alata_, L., and _D. purpurea_, Roxb., are cultivated in gardens, but are not found wild. The Chinese yam, _Dioscorea batatas_ of Decaisne,[300] extensively cultivated by the Chinese under the name of _Sain-in_, and introduced by M. de Montigny into European gardens, where it remains as a luxury, has not hitherto been found wild in China. Other less-known species are also cultivated by the Chinese, especially the _chou-yu_, _tou-tchou_, _chan-yu_, mentioned in their ancient works on agriculture, and which has spherical rhizomes (instead of the pyriform spindles of the _D. batatas_). The names mean, according to Stanislas Julien, mountain arum, whence we may conclude the plant is really a native of the country. Dr. Bretschneider[301] gives three _Dioscoreæ_ as cultivated in China (_D. batatas_, _alata_, _sativa_), adding, “The _Dioscorea_ is indigenous in China, for it is mentioned in the oldest work on medicine, that of the Emperor Schen-nung.”