Section 8. A tax shall be imposed on marihuana and controlled substances as defined in section one at the following rates:—
(1) on each gram of marihuana, or each portion of a gram, three dollars and fifty cents; and
(2) on each gram of controlled substance, or portion of a gram, two hundred dollars; or
(3) on each fifty dosage units of a controlled substance that is not sold by weight, or a portion thereof, two thousand dollars.