Full Idea
'Tree' is not a sortal concept under which directions fall since we cannot adequately explain the truth-conditions of any identity statement involving a pair of tree-denoting singular terms by appealing to facts to do with parallelism between lines.
Gist of Idea
Entities fall under a sortal concept if they can be used to explain identity statements concerning them
Source
Crispin Wright (Frege's Concept of Numbers as Objects [1983], 3.xiv)
Book Reference
Wright,Crispin: 'Frege's Conception of Numbers' [Scots Philosophical Monographs 1983], p.114
A Reaction
The idea seems to be that these two fall under 'hedgehog', because that is a respect in which they are identical. I like to notion of explanation as a part of this.