The large, white items in the picture are actually clusters of many tiny, white flowers. Those clusters are inflorescences of flowers, not flowers themselves.
By producing tiny flowers grouped in large clusters the plant accomplishes two main goals. First, by having large, bright clusters of flowers, pollinators are attracted from far away. Second, since each individual flower is small and just one of many, if something happens to it, the plant has plenty of other flowers remaining to get the job done.
At the left you see an inflorescence from a carrot plant growing in my garden. Notice how the cluster is made of smaller clusters. The inflorescence is flat-topped, with the small clusters on the outside of the big cluster being on longer "stems" than the small clusters in the center of the big cluster. Such flat-topped inflorescences are known as umbels, and the smaller clusters are umbellets. When umbels consist of numerous umbellets they are said to be compound umbels.
In the picture of an umbellet shown at the right you can see the individual flowers (as well as some ants). To understand what you see you may want to review our Standard Blossom Page. Each "real flower" has five petals, five stamens alternating with the petals, and the ovary is inferior (arising below the petals.) If you look closely at an umbellet in your hand, you'll see that the umbellet's outer flowers have larger and more unequal petals than flowers in the center.
In the picture you can see that the ovary looks like a cylindrical, thickish stem below the petals. That ovary has two long cells next to one another, and each cell produces a single seed. The mature fruit with its two seeds is about 1/8-inch long and has tiny bristles along its secondary ribs.