The ornamental design for plant bowl, as shown and described.
FIG. 1 is a top view of a plant bowl showing my new design;
FIG. 2 is a front elevation thereof, the rear and both side elevations
being the same and, therefore, not shown, these elevations also disclosing
most of the bottom, the center remainder of the bottom being plane and
unornamented and, therefore, not shown; and,
FIG. 3 is a top/front/left side perspective view thereof.