John: Hmm.. I only see upwards-pointing arrows for the middle, vertically aligned arrows. The ones at 45 degree angles are undirected. I wonder why that is?

I just checked -- in my browser, they do all have arrows! I guess something's wrong with my pdf reader. [I'm using Document Viewer, which is probably Evince, in Gnome on Fedora 27] It all makes sense now.