Vascular tissues form interconnected systems of continuous cell files throughout the plant body. All types of vascular tissues differentiate from provascular cells that become recognisable as aligned, narrow cells early in organ development. Confocal laser scanning micrographs show that the emergence of a network provascular strands in early leaf primordia of Arabidopsis thaliana. Strands are ordered in a system of hierarchical classes (1° to 4°) as they branch from each other. Individual strand classes can be selectively manipulated, as they become successively susceptible to experimental interference. A, B: Initiation and elongation of the primary strand (or vein) prior to the initiation of the leaf lamina. C: Formation of continuous lobes of secondary veins is initiated distally. Gradual alignment of cell division planes in a narrow strip indicates the position of proximally added secondary vein lobes. D: The position of tertiary veins can be inferred from cell division patterns within the lobes of secondary veins. Quarternary veins (not shown) follow, as the area within these lobes increases. (From Mattsson et al. 1999; Development 126: 2979-91).
