Vascular Bundle Differentiation and Cambial Development in Cultured Tissue Blocks Excised From the Embryo of Ricinus Communis L.

Abstract

Small interfascicular and fascicular tissue blocks were excised from the hypocotyl of mature castor bean embryos. When cultured on a liquid medium containing mineral salts, sucrose, and kinetin, many of the interfascicular tissue blocks produced a cambium. Fascicular tissue blocks exhibited remarkably little development of vascular bundle tissue, particularly in the morphologicallyapical side of the block. Cambial development was frequently observed in this kind of tissue block, however, and was apparently not influenced by the lack of vascular bundle differentiation. Omission of kinetin from the medium prevented cambial development in both interfascicular and fascicular tissue blocks but did not lead to any further reduction of vascular bundle differentiation in the latter.It is therefore concluded that vascular bundle differentiation and cambial development represent two essentially unrelated developmental processes, both initiated in the mature embryo but each depending for its realization on different specific internal and external conditions. The precambial layer, i.e, the layer(s) considered to represent the initiated cambium, can be identified in the interfascicular zone as the innermost of two cell layers characterized by the relativelysmall size of the cells and nuclei. Attempts to obtain enzyme-histochemical evidence corroborating the special character of the precambial layer have been unsuccesfuI.

The position of regenerating cambia: auxin/sucrose ratio and the gradient induction hypothesis

“…(Undifferentiated tissue may be placed so that the gradient in it is induced jointly by the surface (1) and by adjacent differentiated tissue ( 5)). This * gradient induction ' hypothesis subsequently proved applicable to patterns of vascular differentiation in such diverse situations as the internal cambia formed in relation to internal phloem in wounded Solanaceous stems (Warren , tertiary cambia in Vitis voinieriana (Meyer 1962), regener ating cambia in grafts between petioles and stems (Warren Wilson & Warren Wilson 1963), wound cambia formed in debarked tree trunks (Noel 1968), the normal vascularization at the stem apex of Gleditsia triacanthos (Neville 1968) and interfascicular cambia in the hypocotyl of Ricinus communis (Siebers 1972).…”