Please check this link
Other info at http://www.pfaf.org/database/plants.php?Dioscorea+batatas
Won't it be just as easy to buy the yam roots from either a Chinese or Japanese grocery store and propagate from the cuttings?
See info about growing
A. Sweet potatoes
are started from transplants or vine cuttings rather than from seeds
. Transplants, also called slips, usually grow from bedded roots. A vine cutting is 10- to 12-inch section cut from a vine growing
in the field. Home gardeners can produce a limited number of slips or sections of vine by placing a sweet potato
bud side up in a jar of water and placing the jar in a sunny location. Vines produced can be cut into sections and planted.
and from http://the.honoluluadvertiser.com/article/2006/Nov/24/il/FP611240317.html
are started from vine cuttings or slips. Starter material can be purchased from your local garden shop, or you can sow your own planting material. The easiest way to get started is to purchase a sweet potato
with your desired characteristics (skin color, flesh color, flavor, etc.) from the grocery store. Avoid sweet potatoes
with excessive pest or disease damage. Keep the sweet potato
in a moist area or plant
it a few inches below the soil surface to serve as a seed
crop. As new sprouts emerge, cuttings of 12 inches can be used as planting material.