Hello.
Are limes just green lemons or are they a separate fruit? 
Chris76
They are separate fruit.
Or, rather, they are only the same fruit in the same way they’re both the same fruit as oranges or grapefruit. (All of them are in the genus Citrus, but different species - although closely related enough to hybridize, in many cases.)
To expand - at the community garden I volunteer at, we have lemon trees of various sorts…and lime trees of various sorts. Neither one produces the other, even if we leave the fruit to rot.
-D/a
It is perfectly possible to graft a lemon stalk into a lime tree (or vice versa) to get lemons and limes from the same tree.
Si
There are many varieties of both lemons and limes - for example:
Lemons:
Ponderosa
Eureka
Lisbon
Meyer
Limes:
Bearss
Key
Rangpur
Citrus are completely indiscriminate in terms of cross-breeding. Nearly all the citrus fruit you see in the store are the result of hybridization between the original ancestral strains of citrus fruit. This includes multiple strains of lemons and limes. They’re different strains of fruit, but all just different varieties of citrus.
The importance of the difference between lemons and limes and different subspecies, esp. since they all look similar to the untrained eye, are described in this very interesting essayabout Scott’s journeys in the Antarctica, scurvy, the importance of the proper scientific method and many other things.
Thai uses the same word for both: manao.
Lemons start out green, looking pretty much the same as limes, so it’s a common question. But they are different fruit.