It is hard to tell from the photo.  MY first guess is a scale insect but generally Japanese snowbell is a pest free plant.  Can you poke at the white sacs to see if there is a squishy blob inside (the scale insect which secretes a wax covering) or do little eggs roll out.? Or is there a sticky honeydew on leaves below? Scale insects are sucking insects that excrete excess sugars as honeydew.  However, the leaves look too clean.  The other possibility is a waxy covering on predatory lady beetles.  You can send samples in a crush-proof container t our office at 600 128th St. SE., Everett, WA 98208 or drop them off between 9-4 when Master Gardeners are on duty.  My colleague and I usually look at samples if they can't identify them.

Hi Sharon, thanks for your response. All of the sacs are empty. They were empty when I discovered them. There is no honeydew, deformation, or other signs of sucking insects. The tree looks perfect and healthy other than the white sacs on the undersides of the leaves. I will send a sample when I can. Thanks again. Linda