Standing grain saves both a sheaf and standing grain. A sheaf saves neither a sheaf nor standing grain.  What standing grain is it that saves a sheaf? Any that is not forgotten, even one stalk.

[If] standing grain contains two seahs and he forgot it, it is not shich'chah. [If] it does not contain two seahs but it is capable of producing two seahs, even if it is of grasspea, we view it as if it were a kernel of barley.