Root Wood Differs Strongly from Shoot Wood within Individual Plants of Many Cactaceae

Shoot wood was compared with root wood of the same plant in 120 species of cacti. Monomorphic fibrous wood occurred in both organs in 27 species; in 40 species, monomorphic wide‐band tracheid (WBT) wood occurred in both. But in 23 species, root wood differed strongly from shoot wood; for example, two species had fibrous wood in shoots but parenchymatous wood in roots, and 12 species had WBT wood in shoots but fibrous wood in roots. Thirty‐six species had dimorphic wood (an organ produced one type of wood for a while and then switched to producing a different type) in either or both organs. In eight species, shoots and roots initially produced the same type of wood but then one or the other switched; in 23 species, shoots and roots initially produced different types of wood but then one switched and afterward both produced the same type. But in five species, shoot wood did not match root wood either before or after the switch. Six species had enlarged storage roots as well as ordinary slender roots; wood in one type of root differed from that in the other and differed from shoot wood. The independence of wood morphogenesis in shoots and roots has allowed each to evolve to have characters that are adaptive to each organ.