STRUCTURALLY, PHLOEM UNLOADING IN THE MAIZE LEAF CANNOT BE SYMPLASTIC

Developing longitudinal vascular bundles of the leaf blades of maize ( Zea mays L. cv. W273) were examined with the transmission electron mic roscope to determine the frequency of plasmodesmata between the sieve tubes and their neighboring cells. Of particular interest were the pro tophloem sieve tubes, the first sieve tubes to mature in importing (al l large and some intermediate) bundles. The protophloem sieve tubes, m ost of which lack companion cells, intergrade structurally with the th in-walled metaphloem sieve tubes. Both the protophloem sieve tubes and the thin-walled metaphloem sieve tubes and their companion cells (the sieve tube-companion cell complexes) are virtually isolated symplasti cally from the rest of the leaf, precluding a symplastic mechanism of phloem unloading in the leaf blade of maize.