---
title: "Growth and structure of exogenous tree"
subject: "Civil Engineering Materials"
chapter: 7
key: "semester_3/civil_engineering_materials:exogenous-tree-structure-sketch"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/exogenous-tree-structure-sketch
site: BCT Engineering
---

# Growth and structure of exogenous tree

Civil Engineering Materials · Chapter 7: Timber

- Asked in: 2081 Baisakh, 3 marks; 2075 Asoj, 3 marks; 2073 Shrawan, 4 marks; 2074 Chaitra, 3 marks; 2074 Asoj, 3 marks; 2059 Poush (old course), 7 marks; 2065 Shrawan (old course), 6 marks (asked 7 times)
- Video: [Cross section of Tree || Timber || Structure of timber](https://www.youtube.com/watch?v=esIcNQVtlHo) — Civil Engineering ZoNe
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/exogenous-tree-structure-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-7

## Question

Draw a neat labelled sketch of the cross-section of an exogenous tree (timber log) and describe the macrostructure/microstructure, growth components and their functions.

## Short answer

Exogenous trees grow outward by adding rings. The cross-section shows pith (soft centre), heartwood (strong, decay-resistant, used for timber), sapwood (living, weaker), cambium (growth layer), annual rings, medullary rays (radial food and water transport), and inner/outer bark. Wood cells are fibres and vessels in hardwoods, tracheids in softwoods.

## Answer

**Exogenous trees** (outward growers, such as sal, teak, pine) increase in diameter by adding new layers of wood each year under the bark. They provide almost all the structural timber.

#### Cross-section
```text
         _______________
       /  ___________    \
      /  /  _______   \   \   Outer bark
     |  |  /  ___  \   |   |  Inner bark
     |  | |  ( o )  |  |   |  Cambium
     |  | |  pith   |  |   |  Sapwood
     |  |  \_______/  |   |  Heartwood
      \  \___________/   /   Annual rings
       \_______________/    Medullary rays
```

#### Macrostructure (components and functions)
1. **Pith (medulla):** soft central core; it is the first growth and has no strength.
2. **Heartwood:** dark, dense inner rings formed of dead cells; it gives strength and resists decay, so it is used as timber.
3. **Sapwood:** light, outer, living rings which carry sap from roots to leaves; weaker and less durable.
4. **Cambium layer:** thin layer of living cells between sapwood and the inner bark which forms new wood inward and new bark outward every year.
5. **Annual rings:** concentric rings of one year of growth each (spring wood is light and soft; autumn wood is dark and dense); counting rings gives the age of the tree.
6. **Medullary rays:** radial lines from the pith to the cambium which carry sap and store food, and bind the rings.
7. **Inner bark (bast):** carries food from leaves to the tree.
8. **Outer bark:** protects the tree from weather and insects.

#### Microstructure
Under the microscope, wood is made of elongated cells (fibres, tracheids and vessels in hardwoods) with hollow cavities, whose cell walls are mainly cellulose and lignin. Softwoods have tracheids (about 90% of the volume) that give strength and carry water; hardwoods have vessels (pores) for water, and fibres for strength.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do exogenous trees increase in diameter?
   A. By growing from the centre outward only
   B. By growing inward from the bark only at the pith
   C. By lengthening the pith
   D. By adding new wood layers each year under the bark
   Answer: D. Exogenous trees add annual layers under the bark. (Key point: Exogenous growth)

2. What is the function of the cambium layer?
   A. Forms new wood inward and new bark outward every year
   B. Stores food in the centre of the trunk
   C. Protects the tree from insects
   D. Gives the dark hardness to the core
   Answer: A. The cambium is the living growth layer. (Key point: Cambium function)

3. Which statement is correct?
   A. Sapwood is the dead inner core and heartwood carries sap
   B. Heartwood is dark dead inner wood giving strength; sapwood is the living outer wood carrying sap
   C. Both carry sap equally
   D. Heartwood is the soft central core with no strength
   Answer: B. Heartwood is strong and durable; sapwood carries sap. (Key point: Heartwood vs sapwood)

4. What can be learned by counting annual rings?
   A. Moisture content
   B. Species hardness
   C. Age of the tree
   D. Height of the tree
   Answer: C. Each ring is one year of growth. (Key point: Annual rings)

## Related in Civil Engineering Materials

- [Defects during growth phase of trees](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Atimber-defects-growth-phase) — Civil Engineering Materials, chapter 7

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