I claim the ornamental design for a plant marker, as shown and described.
FIG. 1 is a front perspective view showing my ornamental design for a plant marker;
FIG. 2 is a rear perspective new showing my ornamental design for a plant marker;
FIG. 3 is a rear elevational view showing my ornamental design for a plant marker;
FIG. 4 is a front elevational view showing my ornamental design for a plant marker;
FIG. 5 is a right side elevational view showing my ornamental design for a plant marker, the left elevational view being a mirror image of the right side;
FIG. 6 is a top plan view showing my ornamental design for a plant marker; and,
FIG. 7 is a top and front perspective view showing two separate plant markers, each bearing my ornamental design for a plant marker.
The plant marker decals shown in broken lines in FIG. 1 and 7, as well as the plant name in FIG. 1, and the plants and the plant marker shown in broken lines in FIG. 7, are all shown for illustrative purposes only and form no part of the claimed design.
The broken line showing portions are included for the purpose of illustrating environment and form no part of the claimed design.