Algae come in a variety of shapes and in varied colours due to their different photosynthetic pigments. Algae can be unicellular and microscopic or colonial forming plate-like colonies, thread-like filaments, net-like tubes, or hollow balls. Many planktonic algae species bear horns, ridges or wings to increase their surface area to volume ratio which not only increases their ability to obtain scarce nutrients from the environment, but also protects them from herbivores. Some multicellular brown algae form branched filaments or foliose plants many metres long with complex anatomy. Individual diatoms range in size from 2 microns to several millimeters, although there are only a few species that are larger than 200 microns. Some algae, like diatoms, are encased in a siliceous cell wall which takes the form of a box and comes in many unique and beautiful forms.