Vascular Development—Spatial Arrangement

One of the evolutionary innovations that have made it possible for plants and animals to attain large sizes, invade the land, and exist in the presence of considerable environmental stress was the development of vascular tissue. Much attention has been devoted to variations on the primary themes that have evolved through time, but our knowledge of the control of differentiation of these specialized cell types and tissues from ground parenchyma is extremely meager. 

The problem is not easy to work on experimentally. Vascular development involves the differentiation of parenchyma cells in a developing organ into cell types very different from their neighbors. This occurs in highly symmetrical configurations within a matrix. Key questions for solution are what initiates, regulates, and maintains the spatial distribution of vascular cell formation within the stem, leaf, and root  

When one examines transverse sections from near the apex of a vascular plant s stem, one cannot help but be impressed by the beautiful symmetry of placement of the vascular bundles. In stems it appears that leaves play a key role in determining the location, kind, and amount of vascular bundle develop- 

Hormone specialists generally agree that young leaves are a source of auxin and that the main plant hormone involved in vascular bundle induction and differentiation is auxin. In one of the few quantitative studies made on this point Jacobs and Morrow (1957) observed that the rate of production of xylem strands in Coleus petiole bases was directly correlated with the amount of auxin collectible from the bases. 

For definitive testing of the totipotency concept, single cells or protoplasts must be picked axenically from the culture medium and placed on a suitable growing medium then through manipulation of the mineral, vitamin, amino acid, and hormonal components of the medium cause development of a tissue (usually callus) that gives rise to embryoids that in turn develop roots, stems. 

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