Of course it can be taxed. Not everyone is going to want to grow it. Just like any of us cigarette smokers could probably grow tobacco, while it would be illegal, mostly none of us do it. There would be companies growing it for commercial value, companies selling seeds (legally), etc. Not everyone has the patience and will to grow good pot, or any at all. Just like we could all probably take the time to learn how to cook gourmet dinners, we'd rather not and go out to a gourmet restaurant. So yes, it can be taxed.

deftdrummer: tens of millions of people smoke cannabis often in the U.S., just thought I'd let you know that, because I think you're a bit wrong on there being far less people interested in smoking. While I don't think that there will be much, if any, permanent increase in usage if legalized, I do think more people would be open to try it at least once, or do it occasionally. For the most part, I think the demand would be just as high as tobacco. After all, tobacco is unhealthy. Cannabis is not such a destructive habit.