The ornamental design for a holder for a stem or tube, as shown and described.
FIG. 1 is a perspective view of a holder for a stem or tube, showing the new design in a flat configuration;
FIG. 2 is an alternate perspective view showing the design in a deployed configuration;
FIG. 3 is a top plan view of FIG. 1, the bottom plan view being a mirror image thereof;
FIG. 4 is a left-side elevational view of FIG. 1, the right-side elevational view being a mirror image thereof; and,
FIG. 5 is a front elevational view of FIG. 1, the rear elevational view being a mirror image thereof.
The broken lines showing a cup with water and flowers in FIG. 2 depict environmental matter while the invention is in use, and form no part of the claimed design.