Why is Jesus using wheat sufficient to say that we must always use wheat? (I am sure the biological make-up of the strains of wheat used then and of those used now are different anyway).

It's not merely that Jesus used wheat. It's that the Church has always understood that when He said, "Do this," part of what "this" entailed is using wheat bread (and grape wine). It is an interesting conundrum how much wheat would have to change to be a truly difference species (in the philosophical sense as well as the biological), but I don't see much reason to think that it has undergone such a change in the past 2000 years.

The human body has changed also over all those years but wheat remains the most important staple in the global human diet. It's contains the highest vegetable protein of all grains and has continually held that elevated place as the primary staple in the human diet. It equates really well to the nature of the food that Jesus gave us in the sacrifice of His body and blood as the primary spiritual staple known to mankind.

Mmmmmmmmm ... grain vegetable protein.

All this talk about vegetables and stuff is making me awfully nervous.

I think you're safe, gherkin. Cucumbers don't make a significant contribution to the human diet really. They're just there to make the dish look pretty and mostly get scrapped in the bin after dinner's done.

I don't see much reason to think that it has undergone such a change in the past 2000 years.

- that doesn't seem very certain. If it has undergone such a change in the past 2000 years, would it matter?