How does the shape of the anther allow it to hold pollen?

Think of the anther as a long tube like sack or bag. The tube has a seam down the center, top to bottom making it two tubes joined together in the middle. The sacks are sealed closed at the top and bottom. The sacks seem empty at first but then the pollen grains start forming. As the pollen grows and is green it is held in the bags until the pollen matures and turns yellow, at this point the pollen splits the seams of both sacks (the two sides of the anther) and the mature pollen is spilled out to fertilize the flower or to be carried away by the wind or by insects or birds like hummingbirds to fertilize other flowers.