XVI International Botanical Congess

We investigated the continuity of vessels through the stem nodes of maize to find whether particles (e.g. bacteria) or air bubbles (embolisms) can pass between internodes. The simple, parallel vascular bundles of the internode branch prodigiously on entering the node. SEM of retted, fully hydrated and air-dried nodes showed each xylem branch composed of much-pitted, short vessel elements with frequent end walls, linking to each other and to the main bundles. Optical microscopy of macerates showed unusually-shaped elements modified to accommodate linkages with several branches, 2-, 3- and 4-way connectors. Dilute suspensions of 0.2-?m latex particles passed through a node in <3% of the main bundles. None passed through 3 nodes. Their passage was blocked by end walls. Each node therefore forms a filter which prevents access of bacteria or embolisms to the inserted leaf and the next internode.