I bought a tuber that I was told was "Chinese yam". I looked that up, and the most common interpretation would be Dioscorea batatas (aka D. opposita) However, upon slicing it open I was shocked to see an intense purple color. This color (on the cut surface) faded away over the next several hours. If it's a yam, the coloring would suggest P. alata.

I understand there is also a purple sweet potato. In the absence of vegetative growth, is there a way to tell the sweet potato and yam tubers apart?

Thanks for your comments.

However the tuber I have is neither the species/variety of yam nor the sweet potato that you describe.

Dioscorea alata (at least some varieties) has intensely colored purple flesh like that I see. Dioscorea batatas does not. I believe, but I'm not 100% sure, that Dioscorea alata is the only one of the genus that has flesh of this color. Certainly it is by far the most common species of the color. It is also known as Ube.

As far as availability, the market I found it in was not technically "Asian" but it is in a neighborhood that is more than 50% Asian, mostly Chinese. It has lots of fruits and vegetables that I'm not familiar with. And the name "Chines Yam" is often applied to D. alata, although not as frequently as D. batatas is. However D. batatas would not have purple flesh.

If it were a sweet potato, from what I understand it's most likely Okinawan puple sweet potato (I don't know of any other purple varieties, and nothing I've looked into suggests that a different variety that is commonly sold has purple flesh).

As far as size: yes yams CAN grow big. Is D. alata generally sold as tubers greater than 4 lbs? No. From "The South American Table" by Maria Baez Kijac:

"The yam (Dioscorea alata), called name in South America, is native to Africa but now grows in many tropical regions of the world. Yams are usually the size of large potatoes, but some may weigh up to 100 pounds."

Just to complicate things, I'm positive that the tuber has been abraded so that it looks paler in color--there are darker, scaly regions where it looks like the abrasion process didn't (and often couldn't) reach. Lightly colored regions only exist where it looks like regions of the skin have been scratched off. Imagine dragging you fingernails multiple times across the surface--the parallel markings would look like this. I have a regular yam right next to it--it has paler, slightly yellow skin that is smooth. It's not the Okinawan variety--it has white, slightly yellow flesh.

So what it boils down to is whether the purple colored tuber is the purple colored yam (D. alata or Ube) or the purple colored sweet potato (Okinawan purple sweet potato). I've spent quite a while looking and haven't found the answer. And again, this is in precisely a neighborhood where you might expect to find real Dioscorea, not just mislabeled sweet potatoes.

Unfortunately that's the D. alata photo that keeps popping again and again. I guess I don't know what a range in shapes might look like--I can't even find the weight of that tuber--surely that will have some bearing on the overal shape (and will also reflect the age to some degree). The tuber I have (but what's the relative scale? where are other examples?) does not have that shape. It does appear that the darker skin was abraded off--it looks like a bunch of tiny fingernail marks all over the tuber except on those areas that were missed, which are dark in color.

As far as availability, you could also say that Durian, Jackfruit and Rambutan (not to name dozens of other fruits and vegetables whose names I don't know) are not generally available; they are around here. Ube, I understand is a largely Filipino dish. The largest Filipino-American community outside California is in New York city. Our county (Santa Clara County) alone has more Filipinos than New York City. There must be a demand for the tuber here.

I was going to say that I'll just have to wait till it starts growing. You point out, of course, that it may no longer be viable. I also read this:

http://www.hort.purdue.edu/newcrop/proceedings1990/v1-424.html#Yams

"Tubers have a distinct dormancy period, which can be extended with curing and the application of gibberellic acid. This makes yams ideal for long distance shipment and export. Alternatively, this causes large fluctuations in availability of fresh yams, since the crop is not planted until the dormancy period, which coincides with the dry season, has ended."

The rainy season in the Phillipines starts in May (I have no clue where this tuber was grown).

But this gave me at least some hope:

from "Pachyforms: A Guide to Growing Pachycaul and Caudiciform Plants, by Philippe de Vosjoli

"If you receive summer growing plants in winter you can often induce growth and rooting by placing them on heat mats."

So bottom heat it is.

sweet potato (Ipomoea batatas (L.) Lam.)
http://www.knaes.affrc.go.jp/sporf/no08/m4.html
http://plants.usda.gov/java/profile?symbol=IPBA2

I guess I don't see how that resolves the ambiquity between the two types of purple tubers--yam (D.alata) vs. sweet potato (Okinawan). Am I missing something?

Another source of confusion--there is often a light colored ("white")layer between the skin and the purple flesh in the yams(like in my tuber). This layer would be necessary if the skin can be abraded without exposing the purple flesh, as in my tuber. However this is not absolute. An obvious interpretation is that people are confused and the tubers are often misidentified.

How about the rather obvious fading of the purple color at the cut site of the tuber (towards a brownish color). Is that seen with only one or both? I have a cut sweet potato (white fleshed) next to me that has essentially not changed over several days. What about a purple sweet potato--or even a white yam?

It would be much easier to identify if you have a picture of the tuber :-)

Here is the best pic of ube I could find
http://www.stuartxchange.com/Ubi.html

If you google images Dioscorea alata tuber there's a lot of pics of various D. alata tubers.

Differences: skin sweet potatoes have thinner, smoother skin while yams will be darker, thicker, and more fibrous, shape -- the yam will be bumpier and more irregular, and the water content of the flesh. Yams are very dry, sweet potatoes are moister and stickier.