The key difference between heartwood and sapwood is that heartwood is found towards the centre of the trunk and contains inactive secondary xylem, whereas sapwood is found near the cambium and contains active secondary xylem.
Heartwood and sapwood are composed of secondary xylem after years of secondary growth. It’s important to know about the secondary growth in plants to understand how heartwood and sapwood form in plants.
CONTENTS
1. Overview and Key Difference
2. What is Secondary Growth
3. What is Heartwood
4. What is Sapwood
5. Side by Side Comparison – Heartwood vs Sapwood in Tabular Form
6. Summary
What is Secondary Growth?
After primary growth lateral meristem becomes active and results in the formation of secondary permanent tissues. This is called secondary growth. The lateral meristems are the lateral vascular cambium and cork cambium. They are formed only on dicots. In monocots, there is no cambium. Therefore, there is no secondary growth. As a result of secondary growth, there is an increase in thickness or girth in stems and roots. In the stem, the intrafascicular cambium becomes active and cut off cells to the outside and inside. The cells that cut off to the outside become the secondary phloem. The cells on the inside become the secondary xylem.
In the meantime, parenchyma cells between adjacent vascular bundles also become meristematic and form the interfascicular cambium. The intrafascicular cambium and the interfascicular cambium join to form a cambial ring which is the vascular cambium. The interfascicular cambium cuts off cells to the outside and inside. The outside cells become secondary phloem and inside cells become secondary xylem. The cambium contains fusiform initials and ray initials. Fusiform initials give rise to normal xylem and phloem. Ray initials give rise to parenchyma which forms medullary rays. The secondary xylem is continuously being pushed towards the pith as new secondary xylem is formed. The xylem that is pushed further away becomes inactive soon and contributes to the formation of wood.
What is Heartwood?
In many perennial dicots, the cambium is active throughout life. It continuously cuts off the secondary xylem to the inside. The new secondary xylem formed is always found near the vascular cambium and the older secondary xylem is pushed towards the centre. After some time, the older secondary xylem becomes inactive, and certain changes take place. The parenchyma in medullary rays becomes dead. Therefore, there is no food or water in this part. Tannins, oils, resins and gums get deposited on the walls. The cell cavities too get filled with these substances. The xylem vessel cavities get partially blocked by ingrowths of adjacent parenchyma cells. These ingrowths are called tilloses. This part of the secondary xylem or wood becomes darker in colour and is called the heartwood.
Figure 01:Heartwood and Sapwood
Heartwood is used to make furniture and other things because it is hard and not easily attacked by microorganisms. This is because there is no food and water and the presence of tannin and resins.
What is Sapwood?
The active secondary xylem near the cambium is lighter in colour. There are no tannins or resins or other substances. There is food and water in living cells. This part is lighter in colour and is called sapwood and is easily attacked by microorganisms.
What is the Difference Between Heartwood and Sapwood?
One key difference between heartwood and sapwood is that heartwood is darker in colour while the sapwood is lighter in colour. Furthermore, heartwood contains inactive secondary xylem while sapwood contains active secondary xylem. Also, heartwood contains no food or water, but sapwood contains food and water.
Moreover, the heartwood is not easily attacked by microorganisms, while the sapwood is easily attacked by microorganisms. In addition to these, another difference between heartwood and sapwood is that heartwood is found more towards the centre and sapwood is found near the cambium.
Below is a summary of the differences between heartwood and sapwood in tabular form.
Summary – Heartwood vs Sapwood
The key difference between heartwood and sapwood is that heartwood is found towards the centre of the trunk and contains inactive secondary xylem, whereas sapwood is found near the cambium and contains active secondary xylem.
Image Courtesy:
1. “Combretum apiculatum, hout, Phakama” By JMK – Own work (CC BY-SA 3.0) via Commons Wikimedia