Matching Plant Images Internally

[Jorge_Stolfi]

But consider f21r, for example.  Petersen had already "identified" it as "Polygonum" and you listed it above as You are not allowed to view links. Register or Login to view. (Pa) or knotgrass.  

At first sight the resemblance is remarkable and the identification seems to be a sure thing.

This morning I noticed this plant growing on my driveway:
   
It too seems to match f21r.  The same general shape, the same small leaves growing along the stems.  Indeed it is a better match to You are not allowed to view links. Register or Login to view. than Pa, because
The leaves have rounded tips.
The leaves are paired, instead of alternate.
   
But tYou are not allowed to view links. Register or Login to view. says that my plant is You are not allowed to view links. Register or Login to view. alias chickenweed.  Or maybe You are not allowed to view links. Register or Login to view..   No chance for any Polygonum. And the former is listed as having medicinal uses.  

The only problem with this identification is that both species originated from South America, although it has now spread to half the world.

To be clear, I am not proposing that my plant above is the plant on f21r. I showed it here just to show how shaky are all "identifications" of VMS plants.

All the best, --stolfi