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Bidirectional transport sets up developmental polarity
The early development of gametophytes of the fern Onoclea sensibilis is tightly coupled to the delivery of photosynthate from green filament cells to sink regions (rhizoids), and the delivery of osmotic ions from rhizoids to green cells. Richard Racusen (pp. 227-240) shows that the bi-directional transport of photosynthates and ions is necessary for surgically severed and ionstarved gametophytes to establish polarity and subsequent two-dimensional expansion of the prothallus.
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