Place oven racks at the upper and lower thirds positions and preheat oven to 250 F.
Coarsely grate celery root into a bowl using the 1/3 inch-wide holes of a box grater.
Peel potatoes and coarsely grate into a large bowl. Add lemon juice and toss. Coarsely grate onions into the same bowl.
Transfer to towel, then gather the corners to form a sack. Twist tightly to wring out as much liquid as possible.
Return potatoes and onions into a clean bowl and stir in celery root, flour, eggs, salt, pepper and celery seeds until well combined.
Heat 1/3 inch of oil in a 12-inch nonstick skillet over moderately high heat until hot, but not smoking. Fill a ¼ cup measure (but do not tightly pack) with the latke mixture and carefully spoon into the heated skillet, then fry until the underside is a deep golden color (1 ½ to 3 minutes).
Turn over using 2 spatulas and fry until deep golden on all sides (1 ½ to 3 more minutes).
Transfer to paper towels to drain briefly. Keep warm in a baking dish in the oven.
Repeat frying process until all latkes are cooked.
You may fry latkes 1 hour ahead of serving time.
You may shred the potatoes, onions and celery root in a food processer with the shredding disk.
However use 5 eggs (instead of 4), as the machine will grate more coarsely than hand grating and will require more binding.
Monitor the rate at which the latkes brown. If they brown very quickly, try reducing the heat to