I don't understand where you buy things using plants as currency. In example in this link to the FFXIV database you can see that Mandrakes can be used to obtain certain things. 99 Mandrakes for a Mini Mole minion, and 1 Mandrake for Mandrake Seeds. If you click on the Mini Mole you notice that 99 of a ton of different plants can be used to purchase a Mini Mole minion so doing Mognet quests is not the only way. I already have the minion from finishing the current batch of Mognet quests, but wanted to use the extra Mandrakes from my minion intercrossing to purchase more Mandrake seeds to plant for more minion intercrossing. Note that it says purchase and not via desynthesis. http://xivdb.com/?item/5543/Mandrake