Nissa's Expedition
Sorcery
Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for 
or one mana of that creature's color.)
Search your library for up to two basic land cards, put them onto the battlefield tapped, then shuffle your library.
Illustrated by
Magic 2015#188UncommonEnglishLegality
Rulings
You can tap any untapped creature you control to convoke a spell, even one you haven’t controlled continuously since the beginning of your most recent turn.
When calculating a spell’s total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn’t change a spell’s mana cost or mana value.
If a creature you control has a mana ability with 
in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell’s costs. You won’t be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won’t be on the battlefield when you pay the spell’s costs, so you won’t be able to tap it for convoke.