Why do my plants follow the sun?

Why do my plants follow the sun? It is not like they have a brain and a central nervous system.  They do have cells and those cells need a certain environment to grow properly. So why is it that my Cilantro (coriander) is bent? The cells on the shady side of the plant grow bigger and the cells on the sunny side of the plant are smaller or more compact. This causes the plant to lean towards the sun and because it is sitting in my window, the sunlight is filtering in mainly from one side. How do I help my plants grow straight and tall? Simply turn them around as often as needed. The faster the plant grows, the more often you'll need to do this. I turn my Cilantro about once a day. The second photo should really be first but it will not let me change it on my phone.