Diatoms are composed of two sides, called valve faces.  Over time, these valve faces grow thicker and then split apart, creating two diatoms.  Monoraphid diatoms have a raphe system on just one of their two valve faces.  The raphe system on the one side allows them to move around on the surface of another underwater object, while the non-raphe side can be focused on photosynthesis and other cell functions. 

This monoraphid system makes these diatoms perfect for attaching themselves to underwater plants and other larger algae, including other diatoms.  Diatoms which attach to surfaces in this way are called "epiphytes," meaning "upon a plant."