Nature (1972), 238, 352-353
by Rupert Sheldrake
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Abstract

Almost nothing is known about the establishment of cellular polarity underlying the polar auxin transport system of higher plants. Osborne has suggested that the apical ends of cells derived from an apical meristem by sequential divisions are younger than the basal ends: their polarity and the basipetal transport of auxin are due to this age difference. Sachs in his work on regenerating vascular strands has found that gradients of auxin may be responsible for establishing the cellular polarity and the subsequent transport of auxin in the direction of the initial gradient. Shoot tips and expanding dicot leaves contain relatively high levels of auxin. The basipetal polarity of auxin transport in petioles and stems is therefore associated with basipetal auxin gradients. In grass coleoptiles the greatest amounts of auxin are found at the tip, where basipetal auxin transport is also associated with basipetal auxin gradients."

In monocot leaves which grow by a basal intercalary meristem, the pattern of cell division and of auxin distribution is more or less the reverse of that found in shoot tips. Sequential divisions of the basal meristem presumably make younger the basal ends of cells; and in growing monocot leaves the greatest amounts of auxin are found at the base. The polarity of auxin transport in monocot leaves is therefore of considerable interest.

Hertel and Leopold reported that in the primary leaf of Zea mais, auxin transport was basipetal. No other references to auxin transport in monocot leaves are available and I therefore tested the leaves of a number of species. In every case auxin transport was basipetal.

In leaves of young plants of Avena sativa, basipetal auxin transport took place across the meristematic region at the base of the leaf and also in the leaf sheath, which grows by a basal meristem. Plants germinated and grown in darkness yielded similar results. Less auxin transport was found near the leaf tip than in the younger, more basal parts of the leaf and younger leaves had a greater ability to transport auxin than older leaves. A decline in the ability of cells to transport auxin as they grow older has been observed in a number of other species and tissues.