Assembly for detecting the quantity of plants in a field, having: a first transmitter which can be operated to irradiate first electromagnetic waves in the visible or infrared wavelength range onto a crop in a field, a first receiver which can be operated to receive first waves reflected from the plants of the crop, wherein the first transmitter and the first receiver are mounted together on the harvesting machine or a harvesting device so as to be pivotable about an approximately vertical, slightly forward-aligned axis by means of a pivoting drive, an evaluation device which can be operated to acquire, at different points in a measuring direction extending transversely with respect to a forward movement direction of the assembly and parallel to the ground, in each case a first transit time of the reflected first waves of the first transmitter to the first receiver and therefore to determine a first height profile of the surfaces of the plants in the measuring direction, and to determine the density of the plants by reference to the degree of scattering, dependent on the density of the crop, of the detected transit times of the reflected first waves of the first transmitter to the first receiver in the measuring direction, a second transmitter which can be operated to irradiate second electromagnetic waves onto the crop, a significant proportion of which waves penetrates the crop and is reflected by the ground, and a second receiver which is configured to receive second waves reflected by the ground, wherein the second transmitter and the second receiver are mounted, together with the first transmitter and the first receiver, so as to be pivotable about the axis by means of the pivoting drive, wherein the evaluation device can be operated to acquire, at different points in the measuring direction, in each case a second transit time of the reflected second waves of the second transmitter to the second receiver, and therefore to determine a second height profile of the ground underneath the plants in the measuring direction, and wherein the evaluation device can be operated to determine the quantity of the plants by reference to the first height profile, the second height profile and the density of the plants. a first transmitter which can be operated to irradiate first electromagnetic waves in the visible or infrared wavelength range onto a crop in a field, a first receiver which can be operated to receive first waves reflected from the plants of the crop, wherein the first transmitter and the first receiver are mounted together on the harvesting machine or a harvesting device so as to be pivotable about an approximately vertical, slightly forward-aligned axis by means of a pivoting drive, an evaluation device which can be operated to acquire, at different points in a measuring direction extending transversely with respect to a forward movement direction of the assembly and parallel to the ground, in each case a first transit time of the reflected first waves of the first transmitter to the first receiver and therefore to determine a first height profile of the surfaces of the plants in the measuring direction, and to determine the density of the plants by reference to the degree of scattering, dependent on the density of the crop, of the detected transit times of the reflected first waves of the first transmitter to the first receiver in the measuring direction, a second transmitter which can be operated to irradiate second electromagnetic waves onto the crop, a significant proportion of which waves penetrates the crop and is reflected by the ground, and a second receiver which is configured to receive second waves reflected by the ground, wherein the second transmitter and the second receiver are mounted, together with the first transmitter and the first receiver, so as to be pivotable about the axis by means of the pivoting drive, wherein the evaluation device can be operated to acquire, at different points in the measuring direction, in each case a second transit time of the reflected second waves of the second transmitter to the second receiver, and therefore to determine a second height profile of the ground underneath the plants in the measuring direction, and wherein the evaluation device can be operated to determine the quantity of the plants by reference to the first height profile, the second height profile and the density of the plants.