I planted Romaine lettuce from seed. I have fifty seedlings starting to grow in a tray with the plastic cover over it. The seedlings are spindly and many have fallen over almost lying down. I have them under a glow light but NOT on a heated mat like my tomatoes. They range from 1-2 inches tall.

Any thoughts why these are not spinging straight up. I have a similar issue with the broccoli and cauliflower, however those are in a tray with a heated mat set to 80oF.

Lettuce is leafy and needs lots of lights. I saw a similar thing happen with mine when they were inside and the light was too far away.

Things to consider:
Distance of plants from light?
Is the light more blue or red (You want more bluish tone)?
How long per days are lights on?

Be careful introducing to direct sun. Once you start to move outside, start with them in the shade the first time. If that is Ok, give them only an hour or two of direct sun and start moving up.

Good luck.