Mutant strains lacking pili show severely attenuated pathogenicity. The development of attachment pili may then result in the development of further virulence traits. Some pili, called type IV pili (T4P), generate motile forces. The bacterium has a flagellum at one cell pole as well as pili. UV exposure induces pili formation and cellular aggregation. There are special types of pili involved in bacterial conjugation. Attached to the follicle is a tiny bundle of muscle fiber called the "arrector pili". Type IV pili (non-sex pili) also aid bacteria in gripping surfaces. Cooperation with pili for CR3-mediated internalization is another function of porins. Gomti, Sai, Varuna, Pili, and Basuhi are the five rivers which make its land fertile. Three subcomplexes form the apparatus responsible for assembling and retracting the type IV pili. Fingers do not contain muscles (other than arrector pili). This allows for up to 10,000,000 different phenotypes of the pili. Along the shore, tobacco, pineapple, gourds, pili grass, and scrub trees grew. They are sometimes called "attachment pili".