Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have been secondarily lost (in which case relatedness to stramenopile ancestors is evident from other shared cytological features or from genetic similarity). Stramenopiles represent one of the three major clades in the SAR supergroup, along with Alveolata and Rhizaria. Members of the clade are referred to as 'stramenopiles'. Stramenopiles are eukaryotes; since they are neither fungi, animals, nor plants, they are classified as protists. Most stramenopiles are single-celled, but some are multicellular algae including some brown algae. The group includes a variety of algal protists, heterotrophic flagellates, opalines and closely related proteromonad flagellates (all endobionts in other organisms); the actinophryid heliozoa, and oomycetes. The tripartite hairs have been lost in some stramenopiles - for example in most diatoms (although these organisms still express mastigonemic proteins - see below). Many stramenopiles are unicellular flagellates, and most others produce flagellated cells at some point in their lifecycles, for instance as gametes or zoospores. Most flagellated heterokonts have two flagella; the anterior flagellum has one or two rows of stiff hairs or mastigonemes, and the posterior flagellum is without such embellishments, being smooth, usually shorter, or in a few cases not projecting from the cell. History and the Heterokont problem The term 'Stramenopile' was introduced in 1989. It identified a group that overlapped with the ambiguously defined heterokonts. The term 'heterokont' is used as both an adjective - indicating that the flagella of flagellated cells are dissimilar - and as the name of a taxon. In the latter context, 'Heterokontae' had been introduced 90 years earlier by Alexander Luther for algae that are now considered the Xanthophyceae.: But the same term was used for other groupings of algae. For example, Copeland used it to include the xanthophytes (using the name Vaucheriacea), a group that included what became known as the chrysophytes, the silicoflagellates, and the hyphochytrids. Copeland also included the unrelated collar flagellates (as the Choanoflagellata) in which he placed the bicosoecids (a type of stramenopile). He also included the not-closely-related haptophytes. The consequence of associating multiple concepts to the taxon 'heterokont' is that the meaning of 'heterokont' can only be made clear by making reference to its usage: Heterokontae sensu Luther 1899;