| Abstract: | The vascular system of the stem of Stylobasium was investigated during its primary and secondary phases with both light and electron microscopic methods. It contains collateral bundles arranged in a ring, separated by rays which undergo regular cambial growth. The phloem consists of short sieve elements connected to sieve tubes by simple sieve plates, companion cells of the same length, and phloem parenchyma cells. During their autophagy-like differentiation and maturation, typical of all angiosperms, the sieve elements of Stylobasium have a peculiar feature, whereby they develop and retain form-Pfs plastids (containing protein filaments and starch). The sieve-element plastids of the two Stylobasium species, and of some 100 species belonging to taxa of which Stylobasium had been considered to be a possible member, have been studied by transmission electron microscopy. With the exception of a few species with form-Pcs plastids (containing a single small protein crystal in addition to starch), the great majority of taxa studied are characterized by S-type sieve-element plastids (containing starch only). The presence of form-Pfs plastids in Stylobasium supports its separation into the unigeneric Stylobasiaceae and the placement of this family close to other form-Pfs or form-Pcfs-containing taxa. While other characters would exclude an affiliation to the Magnolianae (form-Pfs plastids in Canella) or Caryophyllales (form-Pfs plastids in Microtea), an association with the form-Pcfs families Connaraceae and Mimosaceae is positively considered and corresponds to their frequent allocation close to the Rutales and Sapindales. Within the Rutales/Sapindales the sizes of sieve-element plastids (average diameter) range from very large (e.g. in the Julianaceae) to comparatively small (e.g. in Aceraceae) and are used to group the families. The sieve element characters of the Coriariaceae (tiny plastids with almost no starch, wide sieve plate pores, copious P-protein) suggest their removal from Rutales/Sapindales into the neighbourhood of the Cucurbitaceae. |
