Bark vs periderm, and which part of the wood is heartwood, annual rings confusing me
Ananya Reddy11th Sample
19 Aug 2026 · 1 min read
I colour coded the secondary growth section (blue for xylem, pink for phloem, green for cambium) and now I realised the colour code is useless because I'm not sure what some of the terms even mean.
- Periderm = cork cambium (phellogen) + cork (phellem) + secondary cortex (phelloderm). I think that's right? But what exactly is "bark"? Is it the same as periderm? NCERT says it's a non-technical term and I couldn't make sense of the line.
- Annual rings: spring wood vs autumn wood. Which one is lighter and which one has wider vessels? Does one ring equal one year?
- Heartwood and sapwood. Which is inside and which outside, and which one still conducts water?
- Lenticels, what are they? In my notes I wrote they are "pores in the bark" but my friend says they are in the periderm.
I'd really like one clear mental picture of the tree trunk cross-section from the centre outwards.
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[+][–]Riya Singh12thSample· 2mo ago
Bark is just "everything outside the vascular cambium". That means periderm and the secondary phloem (and whatever is left of primary phloem, cortex etc. outside it). So periderm is only a part of bark. NCERT also adds early bark (soft) formed early in the season and late bark (hard) at the end of it.
Your periderm formula is right: phellogen cuts off phellem outwards and phelloderm inwards. Lenticels are the lens-shaped openings in the periderm that let gaseous exchange between inner tissues and the outside, since cork is suberised and impermeable.
[+][–]Kavya NaikDropperSample· 2mo ago
Annual rings: in the growing season (spring) the cambium is very active and makes wide xylem vessels, so spring wood (early wood) is lighter in colour and has lower density. In autumn the cambium makes fewer, narrower vessels with thick walls, so it looks darker and denser (autumn wood or late wood). The two together form one annual ring (one dark + one light band). Count them and you can guess the tree's age. That's called dendrochronology. In places with no real seasons, rings are not distinct.
[+][–]Pooja Iyer12thSample· 2mo ago
Centre outward: heartwood is the soft light outer layer that carries water to leaves, and the dark central part is sapwood. Easy to remember because sap = sap flows = inner. (I think? 😅)
[+][–]Rahul DubeyDropperSample· 2mo ago
Pooja, that's the wrong way round. Picture the trunk from the centre: heartwood is the central, dark, dead part, so the heart. It's full of tannins, resins, oils, gums, aromatic substances and essential oils, so it's hard, durable and resistant to microbes and insects, and it does not conduct water. Outside it is the sapwood, lighter in colour and the peripheral part of the secondary xylem, which does conduct water and minerals.
Then: vascular cambium, secondary phloem, phellogen... and the cork outside.
[+][–]Pooja Iyer12thSample· 2mo ago
omg yes, I reversed it, heart = centre. Thanks rahul, I'll fix my notes
[+][–]Mehak AroraDropperSample· 2mo ago
Whole picture from centre to outside, for your colour-code: pith (small bit) → primary xylem → heartwood → sapwood → vascular cambium (thin) → secondary phloem → primary phloem crushed → cortex → phellogen → phellem. And since cambium forms more xylem than phloem on its side, the wood gets bigger and bigger, while the phloem is crushed and shed. Don't forget lenticels cut through the cork to make gas exchange.
[+][–]Ananya Reddy11thSample· 2mo agoAuthor
this is the picture I needed!! redoing my diagram tonight with all these in the right order 🙏
