An injection system for use in injecting a liquid into a tree, the system comprising: a liquid supply assembly comprising: a liquid supply inlet for providing the liquid to the assembly at ambient pressure, an electric pump for pressurizing and discharging the liquid from the assembly at a first pressure above ambient pressure through a liquid supply outlet, a pressure switch connected to the supply outlet and in electrical communication with the pump for selectively actuating the pump when the pressure of the liquid supply outlet falls below said first pressure; an injection gun comprising: an injector inlet in fluid communication with the liquid supply assembly for receiving the liquid from the liquid supply assembly at said first pressure; an injector outlet for discharging the liquid from the injector assembly; a trigger assembly being in fluid communication with the injector inlet and the injector outlet and movable from a first position to a second position; a liquid dosage assembly for providing a metered dose of the liquid to the tree comprising a first chamber for alternately being in fluid communication with the injector inlet and the injector outlet; and a second chamber cooperatively associated with the first chamber that is adapted to contain a gas under a second pressure above ambient pressure that biases the first chamber so that volumes of the first and second chambers change in inverse proportion to one another wherein the first chamber is for alternately receiving the a metered dose of the liquid from the injector inlet at said first pressure when the trigger is in the first position, and providing the metered dose of the liquid to the outlet at said second pressure when the trigger is in said second position; and wherein the pressure switch is set to actuate the pump when the first pressure is greater than the second pressure and/or corresponding to a condition when the first chamber is filled with liquid. a liquid supply assembly comprising: a liquid supply inlet for providing the liquid to the assembly at ambient pressure, an electric pump for pressurizing and discharging the liquid from the assembly at a first pressure above ambient pressure through a liquid supply outlet, a pressure switch connected to the supply outlet and in electrical communication with the pump for selectively actuating the pump when the pressure of the liquid supply outlet falls below said first pressure; a liquid supply inlet for providing the liquid to the assembly at ambient pressure, an electric pump for pressurizing and discharging the liquid from the assembly at a first pressure above ambient pressure through a liquid supply outlet, a pressure switch connected to the supply outlet and in electrical communication with the pump for selectively actuating the pump when the pressure of the liquid supply outlet falls below said first pressure; an injection gun comprising: an injector inlet in fluid communication with the liquid supply assembly for receiving the liquid from the liquid supply assembly at said first pressure; an injector outlet for discharging the liquid from the injector assembly; a trigger assembly being in fluid communication with the injector inlet and the injector outlet and movable from a first position to a second position; an injector inlet in fluid communication with the liquid supply assembly for receiving the liquid from the liquid supply assembly at said first pressure; an injector outlet for discharging the liquid from the injector assembly; a trigger assembly being in fluid communication with the injector inlet and the injector outlet and movable from a first position to a second position; a liquid dosage assembly for providing a metered dose of the liquid to the tree comprising a first chamber for alternately being in fluid communication with the injector inlet and the injector outlet; and a second chamber cooperatively associated with the first chamber that is adapted to contain a gas under a second pressure above ambient pressure that biases the first chamber so that volumes of the first and second chambers change in inverse proportion to one another wherein the first chamber is for alternately receiving the a metered dose of the liquid from the injector inlet at said first pressure when the trigger is in the first position, and providing the metered dose of the liquid to the outlet at said second pressure when the trigger is in said second position; and wherein the pressure switch is set to actuate the pump when the first pressure is greater than the second pressure and/or corresponding to a condition when the first chamber is filled with liquid. a first chamber for alternately being in fluid communication with the injector inlet and the injector outlet; and a second chamber cooperatively associated with the first chamber that is adapted to contain a gas under a second pressure above ambient pressure that biases the first chamber so that volumes of the first and second chambers change in inverse proportion to one another wherein the first chamber is for alternately receiving the a metered dose of the liquid from the injector inlet at said first pressure when the trigger is in the first position, and providing the metered dose of the liquid to the outlet at said second pressure when the trigger is in said second position; and