Well a photo would be helpful, but they might be adventitious roots - tomato plants often have nodules of these, but if they are longer than that, it can mean the plant is growing in less than optimum conditions.

The stems of tomato plants do carry what look like tiny white spikes. They could be immature roots as when the plants are planted out they will produce a second lot of roots just below the surface which can be an advantage especially if the plants are leggy so they can be planted a little deeper.

Ok. Thank you for your reply to my question.