"string of pearl" bubbles really O2?

Just out of curiosity, has anybody ever really checked to see if the bubbles
rising from underwater plants are really O2 - or just CO2?  A couple of
things make me wonder if it might be the latter:

1. I've never noticed these bubbles in a plant tank without CO2 injection -
no matter how intense the light.

2. The way streams of bubbles seem to be emitted from a single point on a
leaf sure seems unlikely.

3.  It just plain LOOKS like the bubbles that rise from the bottom of a
glass of seltzer water!