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The phloem vascular system facilitates transport of energy-rich sugar and
signalling molecules in plants, thus permitting long-range communication
within the organism and growth of non-photosynthesizing organs such as
roots and fruits. The flow is driven by osmotic pressure, generated by
differences in sugar concentration between distal parts of the plant. The
phloem is an intricate distribution system, and many questions about its
regulation and structural diversity remain unanswered. Here, we
investigate the phloem structure in the simplest possible geometry: a
linear leaf, found, for example, in the needles of conifer trees. We
measure the phloem structure in four tree species representing a diverse
set of habitats and needle sizes, from 1 (Picea omorika) to 35 cm (Pinus
palustris). We show that the phloem shares common traits across these four
species and find that the size of its conductive elements obeys a power
law. We present a minimal model that accounts for these common traits and
takes into account the transport strategy and natural constraints. This
minimal model predicts a power law phloem distribution consistent with
transport energy minimization, suggesting that energetics are more
important than translocation speed at the leaf level.
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