Heartwood vs Sapwood
After primary growth lateral meristem becomes active and results in the formation of secondary permanent tissues. This is called the 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 secondary phloem. The cells on the inside become 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.
In many perennial dicots, the cambium is active throughout life. It continuously cuts off 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 heart wood.
Heart wood is used to make furniture and other things because it is hard and not easily attacked by micro organisms. This is because there is no food and water and the presence of tannin and resins.
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 the sap wood and is easily attacked by micro organisms.
What is the difference between Heartwood and Sapwood?
• Heart wood is darker in colour and sap wood is lighter in colour.
• Heart wood contains inactive secondary xylem, and sap wood contains active secondary xylem.
• Heart wood contains no food or water, but sap wood contains food and water.
• Heart wood is not easily attacked by microorganisms, and sap wood is easily attacked by micro organisms.
• Heart wood is found more towards the centre and sap wood is found near the cambium.