Root to/from leaf transportation

The veins are parallel to each other in the corn leaves.  At the base of the leaf, where it connects to the stem at the node, the system becomes much more complex.  The vascular tissue goes horizontal with fusions between the individual veins.  Also, the xylem ‘tubes’, (vessels) have end walls, forcing the water moving up from roots and stem through small pores that act as filters.  The pores are sufficiently small to filter out any particles being carried upward with the water.  Many bacteria and even some viruses are too large to pass through the pores.  Each node of corn, even in the small seedling, has this complexity of the vascular tissue. Root vascular tissue connects with the stem vascular system at the first leaf node.  Whereas an individual leaf may have up to 20 main veins, the node may have 100 horizontal vascular bundles and with fusion of vessels at the nodes.  This redundancy protects the plants from a problem in single vascular bundle or one root branch from blocking transport of water and minerals to the leaves.  Likewise, the movement of carbohydrates from leaves to roots gets distributed to all roots. Water soluble substances such as minerals and toxins can move freely up the plant with water through the xylem but most fungal spores and bacteria are filtered out by the pores.
 
As a consequence of the entanglement of the vascular system at nodes, infection of by a bacterium such as the cause of Goss Wilt or Stewart’s disease usually causes lesions from the point of infection to the end of the leaf but does not cause lesions on other leaves.  However, toxins of fungi such as race 1 of Bipolaris zeicola and race T of Bipolaris maydis can be distributed to other leaves.  A few pathogens, such as those causing crazy top and smut get to the top of the plant by reaching the growing point early in development and essentially get carried along as those cells divide, but that’s another story. The nodal arrangement of vascular tissue keeps most pathogens from becoming systemic.