THE BRANCH ROOTS OF ZEA .4. THE MATURATION AND OPENNESS OF XYLEM CONDUITS IN FIRST-ORDER BRANCHES OF SOIL-GROWN ROOTS

The late metaxylem (LMX) in the first-order branch roots of maize matures slowly. Fully lignified elements remain as individual cells of mean diameter 67 mu m and length about 2 mm for considerable distances proximal from the tip of branches that retain an active meristem (indeterminate branches). Such branches, depending on their age, have from 0 to 45% of the total length of LMX conduits mature and open for unrestricted conduction of the transpiration stream. In determinate branches (those that have lost their meristem), up to 60% of LMX length is open for conduction. The closed portion of the xylem conduits is at the distal end of the branches. In both indeterminate, and the youngest determinate branches, immature, living xylem elements provide the block in the conduits. Later, progressive decay in the determinate branches passes basipetally beyond the immature elements, but the distal ends of the conduits remain closed, plugged initially by mucilaginous materials apparently of root origin, and subsequently by invading microbes. It is proposed that the closed ends of the xylem conduits restrict leakage from the more proximal regions, and also prevent their blockage by embolisms. The presence of mature LMX conduits open for unrestricted water conduction is consistent with the major role in water uptake assigned to the branch roots in earlier studies.