There is a summoning spell for Dryads: Roused to Wrath, on the Branchwraith Warscroll. Thing is, the whole of its text is as follows:
If successfully cast, set up a unit of 2D6 Dryads more than 3" from the enemy, and fully within a Sylvaneth Wyldwood that is within 12" of the caster.
That's it. Nothing about needing to have a unit of Dryads in your army.
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Mind you, I don't even understand what you mean by counting a unit in your overall force but not deploying it. What you deploy is your overall force. "Any remaining units are held in reserve, playing no part unless fate lends a hand." - The Rules
Those remaining units consist of every other model you have. Is someone trying to argue you can deploy/summon a unit you don't have models for? Because if that's what this thread really boils down to, that person can punch themselves in the face and then we can all go home for tea.
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But Regrowth isn't a summoning spell; it's a healing spell.
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Indeed.