title: The anatomy of the chi-chi of Ginkgo biloba suggests a mode of elongation growth that is an alternative to growth driven by an apical meristem.
abstract: The chi-chi of Ginkgo biloba L. are cylindrical woody structures that grow downwards from the branches and trunks of old trees, eventually entering the soil where they give rise to adventitious shoots and roots. Examination of segments of young chi-chi taken from a mature ginkgo tree revealed an internal woody portion with irregular growth rings of tracheid-containing secondary xylem covered by a vascular cambium and bark. The cambium was composed of both fusiform cells and parenchymatous ray cells. Near the tip of the chi-chi, these two types of cambial cells had orientations ranging between axial, radial and circumferential with respect to the cylindrical form of the chi-chi. The xylem rays and tracheids that derived from the cambium showed correspondingly variable orientations. Towards the base of the chi-chi, the fusiform cells and young tracheids were aligned parallel to the axis, indicating that the orientation of the cambial cells in basal regions of the chi-chi gradually became normalised as the tip of the chi-chi extended forwards. Nevertheless, in such basal sites, tracheids near the centre of the chi-chi showed variable orientations in accordance with their mode of formation during the early stages of chi-chi development. The initiation of a chi-chi is proposed to derive from a localised hyperactivity of vascular cambial-cell production in the supporting stem. The chi-chi elongates by tip growth, but it does so in a manner different from organ growth driven by an apical meristem. It is suggested that the chi-chi of Ginkgo is an "evolutionary experiment" that makes use of the vascular cambium, not only for its widening growth but also for its elongation.
authors.affiliation: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, UK. P.W.Barlow@bristol.ac.uk
authors.forename: Peter W
authors.initials: PW
authors.lastname: Barlow
authors.forename: Ewa U
authors.initials: EU
authors.lastname: Kurczyńska
mesh_terms: D020441:Ginkgo biloba / Q000033:anatomy & histology* / Q000166:cytology / Q000254:growth & development*; D018519:Meristem / Q000254:growth & development*; D014934:Wood / Q000033:anatomy & histology* / Q000166:cytology