If you have ever planed a wet board you have probably seen raised fibers (also known as fuzzy grain).  Raised grain is defined in American Lumber Standards for Softwood Lumber as an uneven surface on dressed lumber in which hard summerwood is raised above the seller springwood but not torn loose from it.

The fuzzy appearance of the surface of lumber is due to the summerwood (the outer, harder portion of each annual growth ring) being projected above or depressed below the level of the softer springwood. Raised grain occurs particularly in woods with pronounced difference in structure between the summerwood and the springwood, especially among the softwoods, but has also been observed in wood with such relatively uniform structure as yellow-poplar. In flat-grain lumber, corrugation of the surface is due principally to a crushing of the hard summerwood into the softer springwood beneath it by the planer knives. The summerwood subsequently rises as the springwood cells underneath gradually resume their original shape.  Pounding of the summerwood into the springwood is aggravated by planer knife dullness. In boards planed with a sharp planer, there is typically no evidence–under the microscope–of crushing of the springwood. Other machine conditions, such as the bevel of the knife edge, the heel on the knives, the knife speed, and the pressure of feed rolls, may also affect the amount of raised grain. By the way, wood planed by hand shows very little corrugation.  Pressure washing has the potential to serve the same purpose as a dull planer.