The word ‘phloem’ comes from the word for bark in ancient Greek. It is a parallel system to the xylem which transports water and nutrients up from the roots, but instead transports the products of photosynthesis (‘photosynthates’) from the leaves to the rest of the tree. A big callout to The International Association of Wood Anatomists for the images in this post, contained in this open-access publication.
One of the main photosynthates produced by trees’ leaves is sucrose (maple syrup anyone?), but others found in phloem include fructose and glucose, sugar alcohols and the raffinose family of oligosaccharides (RFOs). A sugar alcohol known as ‘D-pinitol’ has been found in substantial amounts in gymnospermsref and is believed to be the main carbon transport molecule for Scots pine. In addition to sugars, the phloem system is used for signalling and defence throughout the tree (as is the xylem), so plant growth regulators (including auxin, cytokinin and salicylic acid), proteins, minerals and RNA travel in the phloem sap as well. If a foliar insecticide/herbicide/fungicide has been applied and is able to penetrate the pores or stomata (see foliar feeding), and is able to get into the phloem vs staying inside adjacent cells, it will translocate throughout the plant.ref As a result I would not be eating non-organic maple syrup (previously paraformaldehyde was used to reduce microbial attacks on maple trees for syrup product, but this was banned by 1989).ref
There still seems to be quite a bit that’s unknown about how phloem actually works – an article published in 2014 said “Because of the difficulties in measuring phloem function, particularly in trees, we lack a basic natural history and phenomenology of tree phloem”ref and another published as recently as 2021 said “phloem loading strategies in gymnosperm trees have been only tested in three species: P. sylvestris , Pinus mugo and Ginkgo biloba.”ref
But the basic principle is that sugars are created by the process of photosynthesis, ‘loaded’ into the phloem cells (with assistance from adjacent cells) and transported to places in the plant where they are needed, then ‘unloaded’ (but even the mechanism for transportation of sugars in phloem is debated – a famous theory involving ‘osmotically generated pressure gradients’ has dominated but many recent articles point out the lack of data to support it.ref) According to one account, sugars are loaded from leaves into phloem companion cells by active transport (a process which consumes energy) and then diffuse into the sieve tube elements through the plasmodesmata (cytoplasm which is shared between cells via small pores between them). Water then moves by osmosis into these cells (creating the phloem sap), and sugars translocate (move) when sinks (areas of the plant consuming energy) remove sugar and reduce its concentration in the phloem sap.ref
Phloem is also believed to translocate (move from one place in the plant to another) sugars even when photosynthesis is not taking place – eg. in winter in deciduous species.ref In this case the sugars are coming from storage tissues in the rays and roots.
The cells which make up the phloem system in gymnosperms are different to those in angiosperms (similarly to the difference in xylem), but the basic structure for both is that tubular cells, known as sieve cells (gymnosperms) or sieve tube elements (angiosperms), are connected together via pores in their end walls, and the phloem sap ‘flows’ through these sieve cells/tubes.ref
Below is an image of pine sieve cells. The side and end walls are structurally similar, unlike the sieve tubes of angiosperms. The phloem sap flows from cell to cell downwards, through the pores. Many studies reference the fact that sieve cells & tubes contain material which would appear to create a barrier to flow, which calls into question the abovementioned ‘osmotically generated pressure gradients6’ theory.ref
If you’ve read the post about the cambium, you’ll know that there is a constant process of creating new xylem and phloem cells, and in the case of phloem, the most recent does the conducting.ref The conducting phloem usually lasts for one season, but can remain ‘functional’ for one-two years (ie. the cell is still alive, even if it’s not conducting phloem any more). Like xylem, phloem rings are created – see the image to the right of pinus strobus – all of the dark cells are the annual phloem sieve cells which are now non-conducting. The conducting cells are in the lower purple region.
A key difference between xylem and phloem is that phloem cells are living cells. This means that phloem sap must pass through living cells and their membranes in order to flow and this articleref suggests that this mechanism provides a high degree of control for the plant in managing what gets into and out of the phloem system. The phloem passes through holes in the sieve cells known as sieve plates (see pics below both of ficus species, the left hand side shows a transverse section and the right hand side a lateral section).)
In order to create the space for the phloem sap, sieve cells and tubes are missing quite a bit of the normal cell machinery, including a nucleus, vacuole and ribosomes – so they can’t control their metabolism or make proteins. Although they still have some specific proteins (P-proteins – apparently previously known as ‘slime’!ref), mitochondria, endoplasmic reticulum, and sieve element plastids.ref Both types of sieve cells have helper cells alongside which metabolise on their behalf – companion cells in angiosperms and Strasburger cells in gymnosperms.
Since phloem is full of delicious sugar-rich fluid, it can be a magnet for insects, which in turn introduce microbial pathogens including bacteria and viruses.ref Plants produce metabolites to defend themselves against these pathogens, and also induce sieve plate occlusion – basically blocking up the sieve cell or tube where the pathogen is located to avoid it spreading.ref
Both the active phloem and the old phloem which no longer transports photosynthates are together known as the inner bark. Outside these phloem layers is the ritidome or outer bark. You can read more about bark here.
For bonsai there’s really not a lot you need to worry about with respect to phloem, unless you are wiring super tight and cutting off the phloem (but by then your wire will be well embedded in the outer bark).