The Unique Structure of Agarwood: "Interxylary Phloem"

Many thanks to all the experts who sent me agarwood samples for study.  However, to be honest, looking at these samples with completely different appearances, I'm already confused. To prevent everyone from being confused when I post larger pictures later, I'll use my limited botanical knowledge to explain how I understand the external characteristics of agarwood. Please feel free to correct me if I'm wrong; I would be very grateful.

Main Text

Everyone knows the four key elements for identifying agarwood: "phloem, vessels, wood rays, and wood fibers." Why can we identify agarwood using only these four elements?
The key here is: "interxylary phloem"!
Key point: Interxylary phloem, interxylary phloem

Specifically, it's like this:
1. Everyone learned basic botany in middle school. The xylem and phloem of trees are separate; the xylem is on the inside, and the phloem is on the outside. One is responsible for transporting water, and the other for transporting nutrients (please correct me if I'm wrong).

2. Recalling our middle school knowledge, let's delve deeper. As mentioned earlier, most plants only have a thin layer of phloem, located on the outer part. So, it's very strange. If the phloem is only a thin layer, why do we see phloem on the agarwood samples, and why are the phloem and vessels (which should be part of the xylem) intertwined? This involves a unique structure of the *Aquilaria* genus in the Thymelaeaceae family: the "interxylary phloem." As the name suggests, "interxylary" means that this phloem is located within the xylem, unlike other trees where the phloem and xylem are completely separated into distinct layers.  Plants with interxylary phloem are rare, and even now, botanists are unsure whether this represents an advancement or a regression in plant evolutionary history. However, interxylary phloem is crucial for the production of agarwood, which is why only a few tree species can produce it (see the image for more information related to interxylary phloem);

Plants with interxylary phloem are usually tall trees; it's rare to hear of vines needing such a powerful structure to cope with survival crises. Therefore, generally speaking, if a piece of material possesses all four characteristics of agarwood's appearance and also emits a fragrance, it can be preliminarily identified as agarwood;