Trees are some of the largest living organisms on the face of the planet and they distribute tasks spatially according to which locations can best handle the work. While much of the tree is energy consuming (roots, sprouts, new branches, etc.), the leaves are energy producing. They convert water, carbon dioxide, and sunlight into high energy sugar (and oxygen!) through photosynthesis.
To help coordinate these diverse tasks, trees have an elegant circulatory system which, to good approximation, consists of two parallel tubes, the xylem and phloem, which carry water, minerals, sugar, and other nutrients to and from the leaves.
Xylem brings water, and minerals upwards to the leaves while phloem transports energy from the leaves to whatever parts of the tree are in need. Water in the xylem feels a constant upward pull due to water evaporating from the leaves (transpiration), and also due to osmotic pressure from the root, from water in the soil.
On top of root pressure and evaporation, sugar accumulates locally in the phloem wherever there are leaves. The central tradeoff of energy production can be summarized thusly:
Bigger leaves produce a bigger photosynthetic output, and contribute higher flow speeds, but they also cost more energy to make. Make leaves too small and energy can't be distributed quickly enough, make leaves too big and they use all the energy.
This balance results in an interesting relationship: shorter trees exhibit a wider range of leaf sizes (they are not flow speed limited, as they're small), while the tallest trees have a very small range of possible leaf sizes. In fact, there is a calculable upper limit on the height of flowering trees, which exists at the point where these two limits collide. Such will be the aim of this set.
Xylem and phloem are always in close proximity, one carrying water, the other distributing sucrose using pressure from its neighbor. They depend upon one another to sustain their flows, and so always co-locate.
The injection of sucrose in the leaves creates a low water potential (more on this later) which draws water in from the adjacent xylem by osmosis through a connecting semi-permeable membrane. This, in turn, creates a local pressure that causes the fluid in the phloem to flow toward areas of low sugar concentration.
Thus, a steady flow of energy is sustained by a sustained disequilibrium of sucrose concentration throughout the tree.
The overall flow is illustrated in the diagram below, which is narrated by famed naturalist David Attenborough.
In any case, all of this conspires to create a constant flux of high energy sucrose water (hereafter referred to as "sap") from the leaves to sites of sucrose consumption.
Illustrations by Maxicat Rhododendron