Vascular differentiation/bundles

As a plant grows in diameter, secondary xylem is formed from the cambium. Auxin has been implicated in the control of both cambial division and the subsequent differentiation of tracheary element [47]. When vascular bundles are broken, parenchyma cells can redifferentiate into tracheary elements and restore the functional bundles this occurs in response to elevated auxin levels at the wound site [39]. 

A diffuse or weakly differentiated carpel dorsal vasculature, such as in Nitraria and Peganum, is a feature that appears to be relatively pronounced in Sapindales (Ronse De Craene and Haston, 2006 Bachelier and Endress, 2008, 2009). Also, inverted synlateral vascular bundles have been reported in Sapindales, such as in some Burseraceae (Stevens, 2001 onwards Bachelier and Endress, 2009). 

Majority of the previous works on PALF and PALF-reinforced composites utilized fine bundles with diameters less than 100 pm. Mohamed et al. differentiated large vascular bundles and fine strands and determined that the former had a mean diameter of 241.9 pm and was less varying, while the latter had a mean diameter of 72.7 pm and a larger diameter variation as confirmed by visual observation (Mohamed et al. 2010a). They observed that PALF vascular bundles were present and extracted in one lamina, while fine fiber strands were present in a few bottom layers. The diameters of fine fiber strands were decreasing toward the bottom face of the leaf. 

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. 

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