Fig. 15.4-2a and b. Transverse sections of dimorphic cactus wood (Bishop’s cap cactus, Astrophytum ornatum). This cactus grows initially as a short, fat globose plant, with such a wide cortex that turgor pressure alone supports the plant. At that time, it makes wood like that in the lower figure: wood consisting of vessels and wide–band tracheids. Such wood is so soft and spongy that if there is a prolonged drought, the cactus—wood and all—can just shrink down to a smaller volume. When rain returns, the plant and its wood can swell and enlarge.

            Once the plant is about a foot tall, it becomes heavy enough that some fibrous wood is needed for support, and the plant begins to produce wood like that in the upper micrograph: it is mostly vessels in a matrix of parenchyma, but some nests of fibers (arrows) are present, and presumably if the plant became older and heavier, it would ultimately produce wood with many fibers. Both micrographs are of wood in the same transverse section of the plant: the wide-band tracheid wood occurs near the pith, the parenchymatous/fibrous wood is nearer the vascular cambium.