PLANTS CULTIVATED FOE THEIR SUBTERRANEAN PARTS. 77 



the family Dioscoridece, coustitute the genus Dioscorea, 

 of which botanists have described about two hundred 

 species, scattered over all tropical and sub-tropical 

 countries. They usually have rhizomes, that is, under- 

 ground stems or branches of stems, more or less fleshy, 

 which become larger when the annual, exposed part of 

 the plant is near its decay .^ Several species are culti- 

 vated in different countries for these farinaceous rhizomes, 

 which are cooked and eaten like potatoes. 



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,^ 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 wiU serve as 

 a guide, without the absolute necessity of knowing and 

 estimating the botanical characters of each. 



Roxburgh enumerates several DioscoreoB^ cultivated 

 in India, but he found none of them wild, and neither 

 he nor Piddington * 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 culti- 

 vated 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 ^' indicates 

 six wild species, and he adds that D. sativa, L., B. alata, 



' M. Sagot, Bull, de la 8oc. Bot. de France, 1871, p. 306, has well 

 described the growth, and cultivation of yams, as he has studied them in 

 Cayenne. 



' Knnth, Enumeratio, vol. v. 



* These are D. globosa, alata, rubella, fasciailata, fwrpurea, of which 

 two or three appear to be merely varieties. 



* Piddington, Index. 



* Thwaites, Enum. Plant. Zeyl., p. 326. 



