How does that work? Most plants offer nectar, the main energy source for servicing pollinators. And some plants offer copious amounts of pollen, free for the taking. But this plant is stingy. Those dark brown anthers don't have pollen coated on the outside of them. Instead, they hold it tight internally for a "buzz pollinator" bee. Bumblebees and sweat bees are its main pollinators. Bees hold onto the anthers and vibrate their flight muscles without moving their wings. The pollen explodes out of the anthers and covers the bee once they achieve a fast enough velocity.
Senna is a beautiful plant with unique flowers and wonderful leaf texture for a garden. It's also the host for many butterflies and moths including the migrant Sleepy Orange that makes it to our yard some years and starts small temporary colonies. Senna has a unique feature called "extrafloral nectaries". You can learn more about them in this post. It's Senna marilandica.