Fig. 5.3-13. Transverse section of linden tree (Tilia). These two micrographs show wood (at the bottom) and secondary phloem (the upper part), the high magnification shows the cambial region where the wood is adjacent to the phloem. The red bands of cells in the secondary phloem consist of many phloem fibers, which are so narrow and have such thick walls that only a few have their lumens visible as small white dots. Secondary phloem is part of the bark, and bark has several names: “liber” in Latin and “bast” is an out-of-date term no longer used in English, except when bark fibers are being described: these phloem fibers can be called “bast fibers” (used mostly by people interested in textiles). When a xylary fiber is described as a “libriform fiber,” that means it is as fiberlike as these bark fibers. Xylary fibers are believed to have evolved from tracheids, and during the early stages, they still resembled tracheids by having large pits and thin secondary walls. Those that are as fiber-like as possible -- that is, they resemble the fibers shown here in phloem -- they are said to be libriform fibers.