There are two types of flowers- chasmogamous and cleistogamous.

The flowers with their anther and stigma exposed for pollination are called chasmogamous flowers. E.g. peas, beans, etc.

The flowers which are closed and never open are called clesitogamous flowers. So cross-pollination can’t occur in cleistogamous flowers. The anther and stigma in these flowers lie close to each other so only self-pollination occurs. E.g. Oxalis, Viola, etc.