jllmom - can you be more specific about what you mean in saying 'muscle tone'? It is one of those terms you hear a lot but in of itself, it doesn't mean much.
Cycling will not make your legs stronger unless you have some sort of physical limitation. Studies have shown that people who do not ride at all, are actually stronger on average, in terms of outright leg strength, than elite riders (aka TdF, etc). Forces in the endurance type of cycling the vast majority of people do, are very low - no higher than climbing stairs for instance.
If you want stronger legs in terms of how much you can lift and such, then do weight-training. Cycling can help you lose weight, it is also a phenomenal way to gain cardiovascular fitness, and burn fat, but not for gaining strength.
The one type of cycling that could potentially make your legs a bit stronger would be all-out sprints from a standstill.
So it comes down to something like this.....do you want bigger muscles? Then lift weights. do you want more definition? Then ride you bike, other cardio, and have a sensible diet. do you want to be stronger? Lift weights.