Many algae live attached firmly to a surface, such as rocks, macrophytes or other substrates. Species that 'lie flat' and are affixed to a surface are considered to have a prostrate habit of growth.
Image Credit: Sarah Spaulding
Scanning electron micrograph of live cells of Cocconeis placentula growing on a rock. Note the mucilage pads on some of the cells. This diatom has a prostrate growth habit.