of the lines. The distribution of $\Delta V$ listed in Table 1 is shown in Fig. 2, where the lower limits for $\Delta V$ obtained from Gaussian fits are included. For comparison, Fig. 2 also shows the distribution of CO line widths (twice the expansion velocity) of the sample of 65 bright C-rich AGB stars reported by Olofsson et al. (1993); the distribution of the widths of OH/IR stars is similar as discussed in that paper. The difference between PNe and AGB line widths is clear from the figure. High velocity gas is rarely seen in AGB envelopes (median width $= 25$ [kms$^{-1}$]{}), but is the norm in PNe envelopes (median width $\ga 45$ [kms$^{-1}$]{}).

The expansion velocity of the ionized gas in PNe is also larger than the expansion velocity of AGB envelopes, typically by a factor of about two. The mean expansion velocities given for PNe in the catalog of Weinberger (1989) are 20.3 [kms$^{-1}$]{} for \[\] and 22.5 [kms$^{-1}$]{}for \[\], and the corresponding line widths fall near the center of the distribution of CO line widths shown in Fig. 2. In PNe the large expansion velocities of the ionized gas are usually ascribed to acceleration by the 