'59 transmissions have a vent fitting with a cap over it that allows air in and out but later transmissions do have a vent tube that usually runs along the transmission as you can see in the second picture. The picture that I borrowed from Dave Dare's rebuild shows the vent fitting just above the trans cooler line where your tube is. I inverted it as it would look in the car. Do you know what year your transmission is. It should not be leaking as it's above where the fluid would normally be.