The answer to this is all of the above.
Mutualism is relationship that is beneficial to both organisms involved. The relationship between a plant and a bacterium called a mutualism because both of them help each other. Bacteria are involved in increasing the fertility of the soil by fixing atmospheric nitrogen and thus provide plant with nitrogen. In return, the bacteria make their homes in what usually become nitrogen "nodules" along plant roots. The plant gets food, the bacteria gets shelter, everybody wins.
Answer and explanation:
The meninges
There are actually 3 parts—dura mater, arachnoid, and pia mater.
The brain is soft and mushy, and without structural support it would not be able to maintain its normal shape. In fact, a brain taken out of the head and not properly suspended (e.g., in saline solution) can tear simply due to the effects of gravity. While the bone of the skull and spine provide most of the safeguarding and structural support for the central nervous system (CNS), alone it isn't quite enough to fully protect the CNS. The meninges help to anchor the CNS in place to keep, for example, the brain from moving around within the skull. They also contain cerebrospinal fluid (CSF), which acts as a cushion for the brain and provides a solution in which the brain is suspended, allowing it to preserve its shape.
The outermost layer of the meninges is the dura mater, which literally means "hard mother." The dura is thick and tough; one side of it attaches to the skull and the other adheres to the next meningeal layer, the arachnoid mater. The dura provides the brain and spinal cord with an extra protective layer, helps to keep the CNS from being jostled around by fastening it to the skull or vertebral column, and supplies a complex system of veinous drainage through which blood can leave the brain.
The arachnoid gets its name because it has the consistency and appearance of a spider web. It is much less substantial than the dura, and stretches like a cobweb between the dura and pia mater. By connecting the pia to the dura, the arachnoid helps to keep the brain in place in the skull. Between the arachnoid and the pia there is also an area known as the subarachnoid space, which is filled with CSF. The arachnoid serves as an additional barrier to isolate the CNS from the rest of the body, acting in a manner similar to the blood-brain barrier by keeping fluids, toxins, etc. out of the brain.
