The Occurrence of Newly Induced Phloem in Shoot-producing Explants of Root Tissue of Cichorium Intybus L. Var. Sativa Bisch. Cultivated in Vitro

In a lecture given at the International Conference of Plant Tissue Culture at State College, Pa. (U.S.A.) in 1963, Karstens (1964) expressed the hope that more study would be given to the formation of phloem in vitro and to the structure of this newly-formed tissue. The literature contains repeated mention of the formation of phloem by plant tissue cultured in vitro, especially in expiants but also in established strains. It is striking, however, to find that phloem is only mentioned; no structural details are given and, apart from a few exceptions and those very sketchy, no illustrations are given. Camus (1949) has demonstrated in material cultured in vitro that shoots developing on explants of root tissue of Cichorium intybus induce bundles of vascular tissue in the underlying tissue. He reports that these bundles are partially composed of phloem. It should be noted in this connection that: a. except for topographic figures, no pictures of either the newlyformed phloem or that initially present in the explant are given ; b. only transverse sections could be studied because the path taken by the bundles could not be determined externally and it was therefore impossible to make properly oriented longitudinal sections. c. most of the bundles observed ran through the phloem already present in the expiants or along the already present cambium, which must have made it difficult to determine whether the sieve tubes observed really belonged to the induced bundles.