  The phloem vascular system facilitates transport of energy-rich sugar and
signaling 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 cm
(\textit{Picea omorika}) to 35 cm (\textit{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.
