I'm pretty sure the photo I'm adding this question to and the photo inside the question are both Campanula persicifolia. The two plants are volunteers, 9 inches apart in my yard, same conditions, same day (Jan 29) but the leaf shape looks very different to me.

I haven't kept great track, but I think individual plants have taken on both leaf shapes over time. But I'm not sure and I haven't figured out a pattern (age of plants, conditions or whatever).

The leaves on the flowering stalk look (to me) entirely different from either of these. The leaves of the seedlings are yet another shape (rather than miniature version of the mature plant).

A different weed in my yard (that I haven't identified) has the same basic leaf shape and rosette structure as this second photo. But a very close look at the edge of each leaf shows commonality between these two Campanula distinct from that other plant, and it should be easier in Jan, because the other plant apparently vanished all over the yard when overnight lows dipped below 20F. But some of the Campanula stayed green when overnight lows later dipped to negative 10F.

The second photo is just a tiny bit magnified compared to the first. The leaves of the second bulge much wider (they aren't simply a lot shorter at the same width and viewed magnified).

Any idea what is going on in this variation in leaf shape?