Plants infected with a disease of the vascular tissues (the part of the plant that carries water from the roots to the leaves) will often have brownish streaks in the vascular tissues. Picture: Brown vascular tissue in a woody ornamental plant.
The gradual yellowing of foam due to a photochemical reaction. It is faster in sunlight than in artificial light, although it occurs in both. Fresh foam may discolor in the center of the block as a result of thermal or chemical events. (Does not affect physical properties.)