Data from: Scaling of phloem structure and optimality of photoassimilate transport in conifer needles

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 (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.植物韧皮部（phloem）脉管系统负责介导富含能量的糖类与信号分子在植物体内的运输，从而实现有机体内部的长距离通讯以及根、果实等非光合器官的生长发育。该运输流由渗透压驱动，而渗透压源于植物不同远端区域的糖类浓度差。韧皮部是一套复杂精密的物质分配系统，目前学界对其调控机制与结构多样性仍存在诸多尚未解答的问题。本研究针对最简单的几何形态——线性叶片（例如针叶树的针叶）中的韧皮部结构展开探究。我们选取了四种分布于不同生境、针叶长度跨度从1厘米（塞尔维亚云杉*Picea omorika*）至35厘米（长叶松*Pinus palustris*）的树种，对其韧皮部结构进行测量。研究结果表明，这四种树种的韧皮部具有共通特征，且其输导单元的尺寸服从幂律分布。我们构建了一个极简模型，该模型可解释这些共通特征，并同时考量了运输策略与自然约束条件。该极简模型预测的韧皮部幂律分布与运输能量最小化原则相符，这表明在叶片尺度上，能量效率相较于转运速度更为关键。