The Unique Structure of Agarwood: Interxylary Phloem

Thank you all so much for sending me samples of Qi Pi agarwood for research. To be honest, I’m quite confused by these samples that look vastly different from one another. To keep everyone from getting puzzled when I share the full images later, I’ll explain how I understand the morphological characteristics of agarwood with my limited botanical knowledge. Please feel free to correct me if I make any mistakes — I’d really appreciate it.

Main Text

As we all know, four key features are used to identify agarwood: phloem, vessel elements, xylem rays and wood fibers. Why can these four traits alone distinguish genuine agarwood?

The core lies in interxylary phloem!Please note: interxylary phloem, this is the key point.

Let me break it down in detail:

1. We all learned basic botany in junior high school. For ordinary trees, xylem and phloem grow separately: xylem lies in the inner part to transport water, while phloem is located on the outer layer to deliver nutrients. Kindly correct me if I get this wrong.

2. Building on that basic knowledge: the phloem of most plants is just a thin outer layer. This raises a question — if phloem is only a narrow outer layer, why can we see phloem distributed across agarwood? Moreover, the phloem is interwoven with vessel elements, which originally belong to the xylem.

3. This is where the exclusive structure of Aquilaria (Thymelaeaceae) comes in: interxylary phloem. Literally, "interxylary" means this type of phloem grows within the xylem. Unlike most trees where xylem and phloem form completely separate layers, this structure is quite rare among plants. Botanists still debate whether it is an evolutionary advancement or regression. Nevertheless, interxylary phloem is essential for agarwood formation, which explains why only a handful of tree species can produce agarwood. (Relevant references on interxylary phloem are attached in the pictures.)

4. Plants with interxylary phloem are mostly tall arbors. Vines rarely develop such a specialized structure to adapt to survival stress. Therefore, if a specimen presents all four identifying features of agarwood and emits its characteristic fragrance, it can be preliminarily confirmed as genuine agarwood.

Key Terminology Notes (Professional Standard)

1. 沉香：Agarwood (universal academic/industry term)
2. 棋皮：Qi Pi (retained transliteration, a type of high-grade agarwood)
3. 木间韧皮部：Interxylary Phloem (botanical fixed term)
4. 韧皮部：Phloem
5. 导管：Vessel elements / Vessels
6. 木射线：Xylem rays
7. 木质纤维：Wood fibers
8. 瑞香科沉香属：Aquilaria (Family Thymelaeaceae)