Olson, M. E. (Instituto de Biología, Universidad Nacional Autónoma de México, Tercer Circuito s/n, C.U., Copilco, Coyoacán A. P. 70–367, C. P. 04510, México, D. F., México). Wood, bark, and pith anatomy in Pittocaulon (∼Senecio, Asteraceae): water storage and systematics. J. Torrey Bot. Soc. 132: 173–186. 2005.— Anatomy of wood and bark are described for all five species of Pittocaulon, the stems of which consist of a very thick, water storing pith surrounded by a thin-walled cylinder of xylem which is in turn surrounded by a thick, water-storing parenchymatous bark with a wide cortex. The samples examined came from a wide range of water availability, but showed only slight or no differentiation in vessel diameter or other characteristics that commonly track water availability closely. These observations are offered as supporting the hypothesis that the xylem of water-storing plants exists in a selective environment that is in many ways more similar to that of mesic plants than to that of sympatric conventional woody plants. It is hypothesized that the Young's modulus of the small amount of wood present is high, and that the parenchymatous cortex, which has collenchyma beneath the periderm in all species, contributes significantly to the support of the stems. The strata of the chambered pith in Pittocaulon may expand and contract with varying amounts of stored water. Pittocaulon praecox and P. velatum share a lack of sclerified elements in the bark and fusiform cell nuclei. Fibers and druses in the bark may be synapomorphies of the P. bombycophole P. filare P. hintonii clade.
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Vol. 132 • No. 2