An apparatus for a distribution tower of an agricultural machine for dispensing a granular material, such as seed, fertilizer or the like, in a manner assisted by a fluid flow, comprising: - a housing; - a channel structure which is formed in the housing and comprises the following as a respective channel section: - a feed inlet adapted to couple to a feed channel and thereby to receive particles of a granular material to be discharged conveyed by means of a fluid flow in the channel structure; - a dispensing outlet adapted to couple to a dispensing channel and thereby to dispense, in a first operating mode, the particles of granular material to be dispensed conveyed by means of the fluid flow from the channel structure; - a return outlet adapted to couple to a return channel and thereby to return, in a second operating mode different from the first operating mode, the particles of granular material to be discharged conveyed by the fluid flow from the channel structure; and - a bypass fluidly connecting the dispensing outlet and the return outlet; - a closure device, for which - in the first operating mode, a first closure position can be adjusted in which, in the channel structure, a first passage between the feed inlet and the dispensing outlet is open and a second passage between the feed inlet and the return outlet is closed; and - in the second operating mode, a second closure position can be adjusted in which, in the channel structure, the first passage is closed and the second passage is opened, characterized by a bypass closure device adjustable to open and close the bypass; wherein the closure device and the bypass closure device are forcibly coupled to one another in such manner that the bypass is closed by means of the bypass closure device when the closure device is displaced into the first closure position and the bypass is opened when the closure device is displaced into the second closure position. - a housing; - a channel structure which is formed in the housing and comprises the following as a respective channel section: - a feed inlet adapted to couple to a feed channel and thereby to receive particles of a granular material to be discharged conveyed by means of a fluid flow in the channel structure; - a dispensing outlet adapted to couple to a dispensing channel and thereby to dispense, in a first operating mode, the particles of granular material to be dispensed conveyed by means of the fluid flow from the channel structure; - a return outlet adapted to couple to a return channel and thereby to return, in a second operating mode different from the first operating mode, the particles of granular material to be discharged conveyed by the fluid flow from the channel structure; and - a bypass fluidly connecting the dispensing outlet and the return outlet; - a feed inlet adapted to couple to a feed channel and thereby to receive particles of a granular material to be discharged conveyed by means of a fluid flow in the channel structure; - a dispensing outlet adapted to couple to a dispensing channel and thereby to dispense, in a first operating mode, the particles of granular material to be dispensed conveyed by means of the fluid flow from the channel structure; - a return outlet adapted to couple to a return channel and thereby to return, in a second operating mode different from the first operating mode, the particles of granular material to be discharged conveyed by the fluid flow from the channel structure; and - a bypass fluidly connecting the dispensing outlet and the return outlet; - a closure device, for which - in the first operating mode, a first closure position can be adjusted in which, in the channel structure, a first passage between the feed inlet and the dispensing outlet is open and a second passage between the feed inlet and the return outlet is closed; and - in the second operating mode, a second closure position can be adjusted in which, in the channel structure, the first passage is closed and the second passage is opened, characterized by a bypass closure device adjustable to open and close the bypass; wherein the closure device and the bypass closure device are forcibly coupled to one another in such manner that the bypass is closed by means of the bypass closure device when the closure device is displaced into the first closure position and the bypass is opened when the closure device is displaced into the second closure position. - in the first operating mode, a first closure position can be adjusted in which, in the channel structure, a first passage between the feed inlet and the dispensing outlet is open and a second passage between the feed inlet and the return outlet is closed; and - in the second operating mode, a second closure position can be adjusted in which, in the channel structure, the first passage is closed and the second passage is opened, characterized by a bypass closure device adjustable to open and close the bypass; wherein the closure device and the bypass closure device are forcibly coupled to one another in such manner that the bypass is closed by means of the bypass closure device when the closure device is displaced into the first closure position and the bypass is opened when the closure device is displaced into the second closure position.