Hi Bees, are you going to be collecting pollen? Or seeds?

Collecting seeds. I want to prevent cross pollination.

I think most people use the drawstring bags to collect seeds from flowers they have specifically hand pollinated or to prevent them from being picked or deadheaded. To prevent cross pollination, many simply keep the entire plant in a large protective enclosure, and then provide hand pollination. Some plants will self pollinate or wind pollinate, so you don't need to hand pollinate, but I haven't ever needed to guarantee the purity of my tomatoe plants, so I'd wait for someone who knows what they are talking about.

Martha