Why do some plants experience a secondary growth? Why do some plants grow only in height but others grow in height and width? Discover the answers to these questions in this lesson.
Shoot System Review
We’ve previously looked at the basic structures of the shoot system as well as primary growth of the stem. We will now look at another form of growth known as secondary growth of the stem.Before we do, let’s review a few key components of the shoot system, which is the above ground structures of plants, including the leaves, buds, stems, flowers and fruits. Primary growth occurs at the apical meristem and allows the plant stem to increase in length. However, some plants need more than just growth in the length of the stem. We will now look at this type of growth.
Remember that all plant stem growth occurs at the meristems of the shoot system because this is where cell division occurs. There are two types of meristem in the plant stem: apical and lateral. As we just reviewed, primary growth occurs at the apical meristem and increases plant stem length.Secondary growth is growth at the lateral meristem and increases the girth of the stem. This type of growth is only found in dicots and is not found in monocots.
In order to understand why it does not occur in monocots, let’s review the structure of vascular tissue in both types of flowering plants. There are two types of vascular tissue: xylem, which moves water and dissolved minerals, and phloem, which moves food in the plant stem. In monocots and dicots, these structures are organized a bit differently.In monocots, the xylem and phloem are found in paired bundles and are scattered throughout the stem. Remember that monocots are simple flowering plants such as grasses.
However, in dicots – which are more advanced flowering plants such as roses and apple trees – the xylem and phloem are found in rings, with the xylem on the inside and the phloem on the outside. This organization allows for secondary growth of plant stems.
There are two types of tissue used during secondary growth: vascular cambium and cork cambium. Vascular cambium produces both types of vascular tissue – the xylem and the phloem.
Cork cambium produces cork, which replaces the epidermis of the stem. Sometimes, when a plant gets taller because of primary growth, more vascular tissue is needed to transport water and nutrients throughout the plant. In order to provide new tissue, the vascular cambium creates new xylem and phloem. Because these tissues are found together in a ring in the stem in dicot plants, it is easy to create both types of tissue in one location. The xylem is produced on the inside of the ring and the new phloem is produced on the outside of the ring.This creation of vascular tissue is what creates the rings that you see when you look at a tree.
Secondary growth occurs on an annual basis as more tissue is needed for the tree in the spring following a rather non-active growth season in the winter. You can tell a bit about the growing conditions based on the size of the ring. If there was a good growth season with plenty of water and nutrients, the ring of vascular tissue will be larger than if there was a poor growing season with limited water and nutrients. Scientists can even use this information about the size of the rings in a tree to learn more about past climate patterns in an area.Once the plant starts to produce more vascular tissue, the plant is wider. In order to protect this new growth, the cork cambium produces cork to replace the epidermis, which falls off the plant.
We previously talked about epidermis, which is the outer skin of the plant. A way to remember this is to look at the root of the word – ‘dermis.’ When you have an issue with your skin, you may see a dermatologist – a doctor that focuses on skin. Both epidermis and dermatologist contain the root word ‘dermis,’ meaning skin.
The cork cells are box-like and are impermeable to water and nutrients. That is, they keep the water in the environment outside of the tree while also keeping the water on the inside of the plant through preventing water loss.
These new cork cells will eventually replace the epidermis and create a hard outer covering known as bark. The bark provides more protection for the tree than the simple epidermis.
All growth in plants occurs at the meristems. There are two types of growth in the stem: primary, which was previously covered and occurs at the apical meristem increasing stem length and secondary growth, which occurs at the lateral meristems and increases the girth of the stem. Secondary growth is needed by dicot plants such as apple trees because of the structure of the vascular tissue in the stem.
The growth of new xylem and phloem from vascular cambium allows the plant to increase in size because water and nutrients are able to move throughout the plant. This new vascular tissue is then protected by cork cells from cork cambium, which are box-like and prevent water loss. Both the vascular cambium and cork cambium provide new cells at the lateral meristem for secondary stem growth.
When you’re done with this lesson, you will be able to:
- Describe how secondary growth occurs in plants
- Explain the functions of xylem, phloem, vascular cambium and cork cambium