FIG. 33.10

Did woody plants evolve more than once?

OBSERVATIONS Today, the vascular cambium occurs almost exclusively among seed plants, with only limited development of secondary xylem in some adder’s tongue ferns and quillwort lycophytes (Isoetes species). The fossil record, however, shows that other plants, now extinct, also had a vascular cambium.

HYPOTHESIS The vascular cambium evolved convergently in several different groups of vascular plants.

EXPERIMENT AND RESULTS The vascular cambium is recorded in fossils by xylem cells in rows oriented radially in the stem or root. Thus, the giant tree lycophytes of Carboniferous coal swamps (the top photo) had a vascular cambium, as did extinct tree-sized relatives of the horsetails (the bottom photo) and other extinct horsetail relatives. Phylogenetic trees generated from morphological features preserved in fossils unambiguously show that woody lycophytes, woody horsetail relatives, and the group of seed plants and progymnosperms did not share a common ancestor that had a vascular cambium. Moreover, anatomical research shows that the vascular cambium of extinct woody lycophytes and the giant horsetails Archaecalamites and Calamites generated secondary xylem but not secondary phloem, unlike the vascular cambium of seed plants. Living horsetails do not have a vascular cambium, but fossils show that they are descended from ancestors that did make secondary xylem and have lost this trait through evolution.

CONCLUSION Fossils and phylogeny support the hypothesis that the vascular cambium and, hence, wood evolved more than once, reflecting a strong and persistent selection for tall sporophytes among vascular plants.

SOURCE Taylor, T. N., E. L. Taylor, and M. Krings. 2009. Paleobotany. Amsterdam: Elsevier.